I have the os installed to a btrfs drive and a second ext4 drive mounted to my home folder.
I would like to have my timeshift snapshots stored on the second drive. I know I would have to format it as btrfs, but I’m not sure how to set up a subvolume for it. I’m new to btrfs 
If I remember correctly, Timeshift makes snapshots in btrfs-mode only if they are stored on the system partition (btrfs) taking advantage of btrfs’ snapshot feature. Otherwise, if they are to be stored on another partition/drive, they will be in rsync-mode no matter if the filesystem is btrfs or ext4.
Yes. it will only use a drive with a root subvolume. I haven’t been able to set it up properly yet