MacBook Air start to heating up on left side of keyboard when turns on

How full is your drive? Try cleaning it out. I strongly recommend having 1/3 free on the smaller SSD's (256 GB) with the larger ones you only need 1/4 free. Delete your logs and cache files this app makes quick work of it: DiskDoctor

Looking at your Activity Monitor I see LaunchD & UserEventAgent running heavy here. Review this: UserEventAgent process hanging, causing system slowdown

If you are using an older OS you should upgrade it as well as make sure your apps are also up to date.

You'll need to wipe the drive and re-install the OS & updates. Then let it run this way for a while so the indexer task finishes. Once done the system should be stable. At this point I would make a back up as you may want to revert to this state.

Now carefully install one application as a time let it run for a few hours and note if the problem is present reviewing Activity Monitor. Then do the next in the same manner.

What ever you do don't install anything from your data backup! You want everything to be from either App Store or a known good source (Microsoft etc...)

Once all of your standard Apps are installed Make a fresh clean backup again so you have this as your safety.

Now run a good Antivirus & Malware app across your data backup to make sure its clean then copy what you need back over to your drive. You'll need to give the indexer task some time to rebuild its tables your system should be stable now.

If during any step you find the LaunchD & UserEventAgent running heavy like before you'll need to scratch the drive and recover things from your backups.