A few more good ones:

• Fidel - The untold story
• Paul Robeson - Here I Stand
• Malcolm X - Make it Plain
• The Upright Man (Thomas Sankara)
• Grenada - Future coming Towards Us

There aren't many good "theory" documentaries, but as far as some good video intros to Marxism, here are a few.

• Introduction to Marxism by Professor Richard D. Wolff
• Michael Parenti - Reflections on the overthrow of the Soviet Union
• Michael Parenti on US imperialism, and the overthrow of Yugoslavia
• Michael Parenti on anti-sovietism in the US media
• Michael Parenti on Imperialism and Film - Rambo and the Swarthy Hordes
• Karl Marx and Marxism - BBC documentary by Stuart Hall

Since no one else has suggested it, I'd recommend "The target is your brain" a Soviet documentary from the 80s, that talks about how liberal capitalism will promise nice things, but ultimately offers nothing to the people, every problem the documentary mentions has only gotten worse since, so it can feel even more relevant now than when it was made. Because it is Soviet, you do need to make sure your gf won't just dismiss it out of hand as "propaganda" though, it is propaganda, but one of the most important things to teach a baby leftist is to not just dismiss things because they have been labelled with the scary "P" word but to actually listen to the information and analyse it.

https://www.youtube.com/watch?v=VdLNyr3BarE

Loyal Citizens of Pyongyang in Seoul and the Thomas Sankara documentary.

Not sure how suitable it is to a "baby leftist," but I recommend the Chinese documentary "Historical nihilism and the collapse of the USSR" (历史虚无主义与苏联解体), which can be found on Bilibili with Chinese subtitles or on YouTube with English subtitles.

The documentary touches many themes, from the corrosive effect of the denunciation of Stalin (and Lenin) along with the achievements of the Soviet people by post-Stalin Soviet leaders, the creation of a comprador class in the USSR, the actions of western NGOs and "educational institutions" in cultivating said compradors, and the resulting destruction of the USSR. If you do read Chinese, the scrolling comments on Bilibili consistently make the connection to events today.

introductory in english i don't know, but one good about aes is The Haircut, made by an australian couple, they went to drpk to get a haircut and investigate the myth of legal hair styles.

I forget what it's called and can't find it, but it's an old Chilean doc about Allende (banned in Chile for years) made by commies. It's very sober and informative and pretty clearly related all parties involved. It made my gf understand the difference between socialist vs. capitalist police and that nuance is beyond important (the unions striking to harm Allende)

Remember there is c/documentaries! You might find something good there too.

Taken for a Ride - The U.S. History of the Assault on Public Transport in the Last Century - This documentary takes a look at the old public transport system of Los Angeles and follows the step-by-step process by which it was dismantled by General Motors. IMO it's a good one for seeing a concrete example of the actual steps that privatization can take -- GM bought the streetcars after a campaign calling them inefficient/run down etc., then after buying them, let them degrade in quality and service, then replaced them with a supposedly superior bus system. Then they allowed the buses to give poor service, ultimately promoting individual cars over buses and highway expansions as the solution to traffic congestion.

Former CIA Agent John Stockwell Talks about How the CIA Worked in Vietnam and Elsewhere - This interview clip is only 15 minutes long but gives a very concise and specific example of how the CIA manipulates the media by having contacts with reporters and passing them a mixture of true and false stories, basically coming up with bullshit and fake photos that will go viral and spread CIA talking points while the "source" of the information becomes more and more obscured as the story is passed around different news agencies, as well as how the CIA have funded the production of countless books, whose authors were allowed to write whatever they wished as long as they included this or that specific point, and that these authors have gone on to have solid and respected careers in academia.

Cybersocialism: Project Cybersyn & The CIA Coup in Chile - From what I recall it gives a good overview of what happened in Chile. In my opinion, due to Chile's case being so well-documented, it's a case which people without a lot of background knowledge can start to learn about the process of CIA coups from and how it relates to protecting the interests of the bourgeoisie. A viewer of this documentary can then start applying that knowledge to many other cases where a similar pattern comes up (country tries to nationalize industries/resources which are in foreign imperialist hands => economic loan denial/asset freezes/sanctions are implemented by the imperialists & opposition groups and terrorists in the country are funded & coups are orchestrated by the imperialist power.)

The Human Face of Russia - Simply, lots of footage of everyday life in 1980s USSR. As I recall, it was a foreign group going there to film and fact-check about the living standards and learn about various political and social activities of the people. IIRC it was a pretty calm and positive documentary, a good one if you need some time away from more heavy and upsetting topics.

The Weight of Chains - About the breakup of Yugoslavia.

The U.S. School That Trains Dictators & Death Squads - About the School of the Americas.

Gaza Fights For Freedom - About the Great March of Return.

The Lobby - Four-part undercover investigation into Israel's covert influence campaign in the United States.

is yellow parenti a documentary?

"the corporation" is a little old now (early 2000s) but it covers neoliberalism, globalization, and privatization pretty well

Oliver Stone's series "Untold History of the United States" really helped to open my eyes to the depravity of the US when I was first learning about socialist politics. It gets a little socdem at parts, but for the most part is just a really solid accounting of the US's crimes in the 20th century. You should be able to find it streaming online, but pm me if you can't.
"My Brothers and Sisters in the North" is a good look at life in the DPRK, and not just in Pyongyang. This one's on youtube.
The Act of Killing is a very intense look into the perpetrators of the genocide in Indonesia in the 60s. I haven't watched the follow-up "The Look of Silence", but I've heard it's good as well. I think these are on netflix, but pm me if you want.

Thomas sankara: the upright man, loyal citizens of Pyongyang in Seoul, reel bad Arabs, 638 ways to kill castro, Tito and I, the shock doctrine, capitalism a love story

There is one when a famous guy takes some Americans to Cuba for free medical attention but I can't recall the name.

Ways of Seeing by John Berger is a really good series on the cultural values of aesthetics under capitalism. It’s from the 70s, so a bit dated, but it really shows the collision of art and capitalism shaping culture and ideology.

Plastic Pills and Second Thought have a lot of high quality videos exploring theory concepts when you think that is warranted.

Another thing to consider is what interests they might have that could overlap with theory concepts. This will help find topics that are engaging and the materialism can grapple with that to inform perspective.

Wasn't there one on the Black Panthers that covered the assassination of Fred Hampton by the FBI? I'm trying to reach back in my memory but can't remember it. Anyway I think that one helped me on the road a bit.

The Revolution Will Not Be Televised, Exterminate All The Brutes

aside from all the ones mentioned