[Questions around this topic seems to be very popular inFAANG (and non-FAANG) companies. ]

Can someone provide some examples and hints/ links at how to troubleshoot issues for a 3 tier or multi-tier web application.

I know, "Issues" is a broad category. Then again, the issues can be anywhere. I know people are working on various layers of applications and there are different issues. I'm trying to get a sense of the issues faced, how to figure out the issues and how to resolve.

These are the typical layers/ areas where issues come up (as I can understand):

1. Network devices/ Load Balancers/ NICs/ Network Security Groups
2. Web Server
3. Application Server
4. Databases
5. Client side (e.g. debugged using DevTools)
6. Anything else - e.g. Integration with 3rd parties for OAuth

Would love to get a brain-dump of anything you can think of.
Thanks in advance.