I’m ordering my PCBs with smd components assembled. What is the best way to attach the XIAO ESP32s3 Plus after? Is it safe to use a hotplate with solder paste? Or will that damage the already placed components?
Any suggestions are much appreciated! 
The datasheet for each of your board’s components should specify the times and temperatures allowed for reflowing of components.
it’ll be like “270c for 3 seconds max” or something like that.
the best way to connect a xiao would be adding a low profile female pin header to the board, and having that factory installed so you can switch to a different line of xiao’s if the mood takes you