The trick with JLCPCB is to stay within their parts catalog:
https://jlcpcb.com/parts/
They've slowly lifted restrictions (e.g. can do hand soldering now for connectors) but a huge pain in assembly services is ordering parts and then prepping them for the pick and place machines. JLC basically has a stock catalog of parts on reels already on feeders (the basic parts), and a library of parts they're willing to load into feeders if need be (at a fee per part). Since they already are under the same umbrella as a distribution company (LCSC) they simplified some of the purchasing step, that is if its stock. PCBWay is happy to order anything, make more than 30 parts, etc, where as JLC is more "only this, no exceptions"