How worried should I be about this computer having viruses?

Just got on my family’s PC after not being on it for a few weeks. Upon booting it up, it got stuck on a windows update for ~4 hours before surprisingly reaching the Windows 11 sign-in screen. I then checked the protection history as shown in the video, and found that a lot of threats have been detected over the past few weeks. After that, I did an offline scan and it found a few more, then did a quick scan and found another one by the name of virtool:powershell/maleficams.h, which I chose to remove.

For additional context, this is a fairly new PC that my family has had for a little less than 6 months now. However, I have a few younger siblings that often go on the PC and download whatever, and it seems like the computer has found (and presumably fixed) a lot of viruses as a result of the installation of countless random programs. I should probably mention that I know little to nothing about computer safety and viruses aside from common sense. It at first glance seems to me like the PC is one bad installation away from being unusable, but I also feel like it could be flagging suspicious files that aren’t actually viruses/malware. We simply use Microsoft Defender and have no other antivirus software installed (to my knowledge, at least).

So how bad does this look, and is there anything I should do to make sure the PC doesn’t get ruined by something like a virus?