I “believe” that Russia is attacking Ukraine for selfish reasons

Every country's major decision is made for some self-interested reason agreed to by its dominant political factions. This means nothing.

and that China censors access to foreign information using the Great Firewall

Trivially and ubiquitously bypassed with VPNs. China's firewall is not about limiting information, generally, because it just plain doesn't do that. Chinese people know what's going on at least as well as people outside of China. Instead, it is a form of protectionism to stimulate domestic development of (tech) industry. Every industrialized country did this in order to industrialize. Given the uneven development that was imposed on countries outside the rich Western club through centuries of colonialism and imperialism, this is a very good thing and it's working.

Furthermore, it ultimately depends who do you want to believe or there are hard facts from reputable sources that are simply a hidden by the mass media?

The first rule of propaganda is that it's about emphasis. What is the focus, how are events described in one context versus anotger, snd most importantly: why was this article written and not a different one about a different topic?

You mentioned the great firewall, which is an example of China's ruling class (its government, itself led by socialists) imposing a protectionist market control. It is always described as censorship even though if it were, it has always failed at its job. But what of the ruling class in the US? Does it censor? Of course! The ruling class of the US are the bourgeoisie, the people who own and control business, and they don't even need to use the goverment for most of their own censorship. They own the media companies. They decide who gets hired and who does not, starting with the editors. They even make editorial decisions, though this is rarely reported. Is that censorship widely reported on? Does it even get described as censorship? Why does a host of petty dictatorships hiring and firing to bolster a political bent not called censorship? Well, who would tell you? The papers aren't going to do that, even if journalists wanted to. They don't even do media criticism in the first place. Their goal is to make money, not inform. That decision got made by the editors early in: no, journalist, you can't write about the censorship of this and other papers: (1) it's just normal goid editorial decision-making, (2) it will make us look bad, and (3) it will take too much time and money and won't sell papers.

Whereas countries run by socialists, or even just social democrat nationalists, must use the state to achieve their ends, the bourgeoisue in a capitalist country can and use their own dictatorial powers in their businesses to get what they want, then use the government to protect and increase those powers, and rally together to normalize these dictatorships as the exact opposite thing: just private individuals being free.

There is a lot more to media criticism, how economic forces work and impact everything else, history, and dominant bullshit narratives, but they kind of need to be explored piece by piece. Sometimes it is about certain sources being full of shit and just straight-up non-factual (e.g., Radio Free Asia), but more often it's much more subtle, like depending heavily on the reader not knowing history or geopolitics so that uncritically repeating a statement from the US State Department like it's just a fact.