Adafruit does their own assembly (pick and place / reflow) but not sure what PCB providers they have. There are US based PCB fabs.
Cyber City Circuits does small scale assembly in the US and seem like a cool company (ran into them from the Datasheet Digest podcast they put out). But yeah obviously they aren’t going to be able to match a $35 PCBA JLCPCB prototyping order.