They are in the market but they depend on engines and other components manufactured elsewhere...

https://asia.nikkei.com/Business/Aerospace-Defense-Industrie...

They have, but the only potential Dreamliner competitor on the horizon, the C929, won't enter service until 2030 at the earliest.

https://en.wikipedia.org/wiki/Comac#Products

One of the few things China can’t produce as well as the west are commercial jet engines.

Serious response: COMAC, the "Commercial Aircraft Corporation of China" (1) are working on it, but the competitor C929 aircraft (2) is not expected before 2030.

And they have produced smaller aircraft (3), but but the numbers produced so far are not impressive.

In other words, you're correct that it's not "if" but "when". And that it's a hard task, but they are working on it. The Jet engines seem to be the major hurdle.

1) https://en.wikipedia.org/wiki/Comac

2) https://en.wikipedia.org/wiki/Comac_C929

3) https://en.wikipedia.org/wiki/Comac_C919

Temu is in a great position to enter this market. Being deeply connected to the supplier networks, they could easily put together a plane and become a competitor to Boeing.