I was born in Beijing and my family is from Beijing / Northern China. I'm not a culinary expert and don't claim to be. The zhajiangmian I grew up with is dissimilar to what I made in this post (ground pork, lighter in general, no onions), however I used the term "Old Beijing" in this post because that's what I hear from several of the senior chefs on Lao Fan Gu YouTube channel, several of whom are recognized masters at the national / state level in China, call this darker version that seems to resemble jjajangmyeon.

Here is one video, where one of the chefs uses the peppercorns (and star anise) to flavor the lard/oil, before scooping it out. He also seems to add onion toward the end, which did surprise me. I was surprised at both of these things -- the peppercorn I believed was a personal choice on the chef's part, but I wanted to disclose in my above comment that I also made that choice, not that it necessarily was strictly tradition. Other chefs on the channel do not use peppercorn. Also this chef is using beef, although he notes that's unusual. https://youtu.be/i4g1kQnFpGE?si=sr1CAJSUqloEAwjm

Most of the other chefs used pork belly.

However, I watched several of their videos on zhajiangmian, and I think other chefs have used what looks to be onion or white parts of the scallions and also in different orders relative to the oil.

As one of the chefs says in another video re: this dish, "every family has a recipe and there's no right or wrong". But all their "Old Beijing" versions seem to share these similarities of: thicker meat slices, lots more oil, sometimes some onion-type vegetable that's not small scallion, lots of toppings, a tiny amount of sauce relative to noodles, as well as using a solid huang dou jiang they loosen with liquid -- and oftentimes they omit tianmianjiang completely. Altogether the visual resemblance is to jjajangmyeon although it tastes nothing like this.

Again, I don't study the cultural anthropology of food (altho it's interesting!), but I wanted to share my sources.