Go to an Asian market and get some dried red chilis. They're great for adding a kick to stir-fries. You can break them up first, which will release more of their spiciness, or add them in whole. Just a few dried chilis isn't overpoweringly spicy but you can always add more depending on what level of spiciness you like. When stir-frying with them, stir fry them first with the rest of your aromatics before tossing in your vegetables or whatever you're using them. They should add a light spice and nice fragrance. If you like the Sichuan "numbing spicy" profile, add some Sichuan peppercorns.

Another way I cook with chilis is julienning long green chilis and stir frying them with other stuff. With this, the spiciness level really depends on the chilis; some are mild while some (like the ones I used the other day haha) arehot. I sometimes make a variation on stir-fried potato strips, but with julienned green chilis instead of dried red chilis. I've also seen people use both dried chilis and fresh chilis in one dish.

Spicy fermented bean paste is another super versatile sauce in the Sichuan flavor profile. Sometimes when I work with it, I like to lightly fry it beforehand, until the oil stains red. It tends to not be as immediately spicy, but it has its distinct, fermented flavor.

I have a lot less experience with using them, but China also has varieties of pickled chilis. They're especially a staple in Guizhou cuisine. (Side note: I've been wanting to try to get my hands on some Guizhou-styled pickled chilis but because Guizhou food has a lot less international renown it's probably going to be pretty hard to get them where I am in the West, even with a good Asian market nearby as I am lucky to have.)

In general, the provinces in China known for their spicy food are Sichuan, Guizhou, and Hunan. Sichuan is most famous for the "numbing spicy" profile. Hunan tends to be just straight spicy. Guizhou is known for using chilis in many creative ways and for maybe being the spiciest of them all (it's the birthplace of Lao Gan Ma, after all.) I once got gifted some homemade Guizhou fried chili oil by a family friend from Guizhou and in terms of spice level, Sichuan chili oil doesn't even come close. Also, my parents are from none of these places, but thanks to the internet and internal migration within China all of this stuff is increasingly shared across the country and diasporas. Just please don't expect me to dispense advice on how to make authentic Hunan food or whatever, I'm just your average mediocre Chinese home cook.

(It also makes it this article funnier when you consider that a unified "Han cuisine" isn't a thing lol. There are certainly foods that have become popular across the country, but those all started as regional dishes or riffs thereupon. The idea that Dongbei cuisine is the same as Cantonese cuisine is hilarious tbh.)

Hope this helps!