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?
You can use arpwatch. It reports new mac addresses on a given network interface.
Here's my setup. I have a raspberry pi running pihole and unbound. I set that raspberry pi as my primary dns on my router. Now in order to use the internet all devices will make dns queries over wifi (you can use a wired pi as well) to the pi which means it will also see all devices using your wifi and notify you when a new one shows up or if an ip changes mac addresses.
Keep in mind these notifications use smtp (email) and you most likely need a mail server to receive them.
I have a matrix server on my network that has postmoogle (email bridge) that can receive the emails from arpwatch and send them to me as a matrix message