can you recommend any (opensource or trusted) android apps to check who’s using your wifi? or any cmd prompt code to search on the desktop?
a small doubt: can anyone hack your computer through WiFi? If so, How to prevent that (or) How to take measures after you think you been hacked?
Checking the router is probably the only way to see if someone is active on your network.
Can anybody hack your computer? No. Most people only know how to run scripts that are known and patched in most operating systems.
There are skilled people who may be able to create an exploit or find a vulnerability in your computer, but they will mostly target businesses or people they know will be worth it to hack, so most likely they won’t bother you.
Generally if your on your own WiFi, having a WPA-2+ personal password is enough, but the more paranoid may have an IDS/IPS on their home network.
If your out and about, I’d personally use a VPN. I don’t like public ones and like to recommend setting one up on your home WiFi instead.
If you think you’ve been hacked… change your passwords and run virus scans. If you still don’t feel safe, backup your data and reinstall your operating system.