I made a 100gb partition on my drive and selected that when installing. I thought it would just use that 100gb and not touch the rest of the drive but it seems to have done something weird because the windows boot manager I used to have in my BIOS is gone.
Inside AlmaLinux I can see in my disk usage a 372gb volume which is (or was) my windows partition. Is there any way I can get back to it?
Check /boot or /boot/efi in Almalinux. Your bootloader/MBR might have been overwritten or deleted. In dual boot, both Windows boot manager and grub remains in the same directory. If you still have Windows in one of the partitions, then copy Windows boot manager from somewhere or boot a Windows image and recover from it. There are various guides available online: