I would also add that the more you modify the system (PPAs, packages not installed via the package manager, nonstandard partition layouts) decreases the stability of your system and makes it harder to get back to your current system state if something goes wrong. I like to think about it like balancing a tower of blocks as a kid. Mint is the first block, and is very stable, but each additional block makes the system less and less stable. Mint itself is really stable, but if you do weird stuff the Mint devs can’t do anything about it, which puts you in a bad position until you really know what you’re doing.
The Snap store is intentionally left out by Mint, because they don’t like how Ubuntu manages it. This means that even though the Ubuntu version Mint is based on supports Snap, there’s no guarantee that snaps will work with the same stability which .deb/apt and flatpak packages will, because it hasn’t been tested in Mint. I would advise against using it.
Theres no equivalent to Windows Defender on Linux, because it’s like 14 tools in one, and Linux by nature is a lot more modular. If you want something whcch scans files for malware, the tool of choice would be ClamAV. By default it only scans files which you manually tell it to, but you can set it up to automatically scan any file when it’s downloaded. It’s a lot less sophisticated than Defender, but there’s also just not as much malware for Linux (yet), and if you stick to installing software through the package manager and never giving other files execution permission, you should be fine.