Deep system malware detection

This might get taken down put I will attempt to ask here. As an intermediate level PC nerd with no experience in writing programs, can you guys explain how rootkits and undetectable malware work? I strongly believe my PC may be affected with said threats due to high CPU usage (90% with no programs running), battery drain, PowerShell popping up then disappearing, computer not sleeping when the lid is closed (it will be dead by morning) and every single account that is linked to my PC having been compromised. Having gone through every ‘basic’ option other than reinstalling Windows (SFC scan, Malwarebytes, checks at VirusTotal.com etc), how can one detect or remove said silent programs and why is this a thing? What good are these antivirus programs if undetectable malware can hijack system processes? What would an analysis solution be for the average consumer?

Omen 15, Intel I7 9750, 16GB RAM, Nvidia GTX 1660ti, Windows 10 Home always up to date, Bitdefender installed.