Which logs when an app fails?

Hello,
Some apps are not running in latest frappe version (yeah I know, I’m discovering life).
When app get installed but something fails to run (py, js, db?), is there a specific log to look for?
(I know about the files in logs/ but I didn’t find anything throwing an error, app fails silently.)

Additionally, when upgrading (I read it’s a pain), are they best practices?
For instance, 2 local docker stacks running v14 and v15, and a debugging tool or way of proceeding?

If this message made you smile, or cringe, or cry, please share your solutions, not just your pain.