How to create custom drawing symbol
This video should get you started.
https://www.youtube.com/watch?v=TZLOIb-dUp8
PTC Learning connector if you have access:
https://learningconnector.ptc.com/tutorial/1625635536001/create-symbols
Great video from Dave Martin.
Here is my contributiong: https://www.youtube.com/watch?v=zJrea53xtG8
4K Side's.
Here is a lengthy thread about custom weld symbols. From earlier versions, so not sure what has changed:
https://community.ptc.com/t5/3D-Part-Assembly-Design/Weld-Note/m-p/299415