They are just like the leaders. A 6X tippet will have the same diameter as the small end of a 6X leader. That makes it nice to tie them together. Yup, you tie them together, at least a short piece of the tippet is tied to the end of the leader. Briefly for this reason, most of the so called wind knots you get casting happen in the two feet ahead of the fly.
Knots weaken things.
So you tie on about a two-foot length of tippet material. The stuff is not expensive and when it gets too short from knots, or flies, tie on a new length of the stuff. S o there it is.