I believe the following applies.
You can actually type anything you want as a tag, just don’t wait for the option to select something, just hit enter after putting whatever you want in the tag. But it’s not an “official” tag.
If you put a tag in a meme (i.e. I tagged the queen meme LordCheesus and katechuks) you can click on those tags within the meme and it will show any more memes tagged the same.
But here’s the best part. Once a tag has been used a certain number of times (no idea how many it takes) it then becomes a main tag and becomes available to select as you described.