How can I improve my linux security?

I dabble in Linux from time to time. Oddly enough at one time in the past and just today, I've found my system compromised (that is my perception at least). In my opinion I'm no expert in Linux or even an intermediate user, but I am technically knowledgeable enough to know my way around a computer, especially Windows.

Let me explain whats happened.

Some time ago when I was just getting really serious about leaing to use Linux, I was running Fedora from an exteal drive. I do know that I had never visited any shady websites with that particular installation. What happened next was inexplicable to me as a long-time windows user. I noticed that a file on my desktop moved as if someone had click & dragged it.

I wasn't not paying much attention to it at the time. A day or so later I booted the machine up again and witnessed the same thing happen. To me it was as if someone was "in" my machine, dragging and dropping files from or to my machine from some kind of virtual interface invisible to me.

I nuked that installation with a re-format.

I began to get serious about leaing to use Linux just this week. I decided to just run Fedora from within Oracle's VirtualBox. I was partitioning a drive with fdisk (using SUDO instead of SU after adding my account to the sudoer's file) when I noticed that when I looked at all of my partitions I could see my hard disk outside of the VM! I entered another command, and recieved a waing about a AVC denial. As soon as I said "That doesn't look right..." my Windows 10 OS running the VM signaled that Windows defender and my AV solution had been tued off. A bit alarmed, I proceeded to try to get it running again but Windows wouldn't have it. I spent a few hours fixing everything. It got to a point (after shutting down the VM) where whenever I booted and logged in to Windows 10, a BSOD would occur with a "watchdog_violation" of some kind.

This VM needed an update, it had been bothering me for software updates for at least two or three days. As with my Windows experience, getting compromised in a short period of time like that didn't seem likely to me.

I managed to fix it just now and I'm on it running a windows defender scan.

So my question is: how can I prevent someone from compromising my machine in the future? What are some security best practices with Linux and VM's?

I did not use SU, I used SUDO. It was running from within a VM, and I don't know how the connection to the web works in that configuration. I don't know how in the world someone would have managed to get in to my VM and then my outer machine that way.

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