I can't log in into iCloud service on MacBook Air(early 2015)

Sorry to hear you are having problems with your computer. Let's see if I can answer a couple of your issues: If your computer thinks you are logged in but you are asked to log in, log out. Then log back in. Sometimes iCloud seems to get confused. Actually while you might think of logging in as being all or nothing, being logged into iCloud provides you iCloud services such as being able to sync your appointments to the Cloud so your other devices and/or computers can see them. This isn't the same as being logged into the Apple App store. You can be logged into one but not the other. And, neither is the same as being logged into iTunes.

Finder shouldn't freeze on you, obviously. There's a program in the App store called EtreCheck, written by a member of the Apple Support Community, that is helpful in diagnosing problems. If you'd like additional help with your computer, download it and copy the completed report. Then come back and start a new message. Click on the icon below:

in the text box, and then paste the results of EtreCheck into the window.

As for switching to Windows...hate to tell you but OS updates in Win10 are such a hot mess that I currently refuse to accept any - and I used to be part of the Insider group. In the last year I've had to wipe my Windows notebook and restore from a backup more times than I want to think about, all because updates (which used to be 100% mandatory) hosed my computer. Yeah, right now I'm not loving my PC.

my wife and I both have 2015 MBPro computer and we aren’t experiencing any problems. I suggest 2 things. First, run apple diagnostics. How to use Apple Diagnostics on your Mac - Apple Support and run the long test. you want the computer to get warm so the diagnostics can fail from a thermal issue is that is the root cause.

Also, for a few days you might want to restart in Safe Mode and run disk first aid to see if the disk directory is remaining healthy. Good luck, I hate problems like yours, it is maddening not to be able to say, “here’s the problem and I know how to fix it.”