I also had a HDD which had Garuda Gnome installed. After installing KDE on SSD I used KDE Partition Manager to Delete my HDD and then create a new partition.
After creating the partition it is showing up on Dolphin at the bottom left sidebar as Internal HDD. However, I can't create any folders on it. I want to use it for mass storage.
It can be any format, but keep in mind that you have to be root user to write in almost all the partition system, except NTFS.
For more info, refer https://wiki.archlinux.org/title/File_systems
Ext4 is fine, but you might want to investigate BTRFS for the caching and compression. Speeds up disk reads a small amount, which is nice having in my slower, data-containing 10,000 RPM HDD. I like. Or at least gives the feel of it.