Linux

First, calm down! There's pretty much no way you broke your computer doing this, and unless you installed Linux over your C: drive by mistake, then you probably didn't even accidentally lose any data.

I really thought I was doing it right and installing Pop to my second D drive to leave windows alone but somehow it completely broke Windows and I can load into that, only Pop!

Dual-booting is kind of a mess at the best of times. In the future I would recommend backing up ALL of your important data before doing anything involving partitions or OS installed, regardless of whether it's Linux or Windows.

Assuming you didn't install over your Windows install or anything like that, then your Windows partitions and stuff should still be there on your other drive. There are a couple of things you can try in order to confirm that your data is still where you left it.

1. Reboot your PC and press whatever key is needed to enter into your UEFI/BIOS menu OR your boot device selection menu. From there you should be able to try to instruct your computer to boot directly into whatever drive you think has Windows installed on it. If that works, you should still be able to boot into Windows (and then back up your important data!).

2. If for whatever reason you still can't boot into Windows, another thing you can try is to "mount" your old Windows NTFS drive within Linux using something sudo mount /dev/YOURPARTITION /mnt, where YOURPARTITION is whatever NTFS partition your C drive is on (it'll probably be something like "/dev/sdX" or "/dev/nvmeXnY"). Once you've mounted your C drive to /mnt, you should be able to use the cd and ls command to look around your files, at which point you should absolutely make sure that you make a backup copy of everything you care about before you proceed.

(To drive the point home, making a proper backup of your system before doing any OS-level stuff is not only a good idea, but in the future it'll save you a lot of stress and/or heartbreak.)

Note: If mounting your NTFS fails, that doesn't necessarily mean anything has gone terribly wrong. sometimes that can happen when Windows doesn't shut down properly, leaving a flag in the wrong state. If that happens, you can still fix it, but that's a different story.

I guess isn't really my GPU (Nvidia 1080ti I think) so on my 4k monitor everything is blown up and the wrong aspect ratio, cutting off the bottoms of windows

I don't know about PopOS specifically, but I would assume this is because NVidia dropped support for 10-series cards from their most recent Linux driver. I ran into a similar issue with my little 1060-3gb, recently, and it sucks. I solved it by upgrade to a RX 9060 xt, which works flawlessly.

If you're not too afraid of the terminal, you should still be able to access a full-screen Linux terminal (called a TTY or virtual terminal) using Ctrl+Alt+F3. From there you'll at least be able to control you computer and check that everything is still working and all of your data is safe. I don't know enough about Pop to say if/how you can change the resolution or fix your driver situation from there... Ctrl+Alt+F2 usually brings you right back to the desktop, by the way.

I've tried some terminal stuff I've read online already but nothing has worked and I'm afraid to do the purge ~nnvidia command because it said it might turn my screen black and if I can't even get into Pop! then I'm screwed.

I've never seen a driver situation that's so broken that you can't at least get into a TTY using Ctrl+Alt+F3.

Have you tried getting on your phone and visiting the PopOS discord or other chat rooms?