Confused on Mythic +

Ok, I have watched multiple videos and read like 5 sites and still super confused. I got most of it but here is my question:

If i run +11 - +15, I then need to farm 0 - +10 to increase the “power” of the +11 - +15 items I get?

If i run +16 - +20, I then need to farm +11 - +15 to increase the “power” of the +16 - +20 items I get?

No way that can be right but that is my understanding.

Technically you don’t need to, once you cap out on a higher crest then the game will start giving you the lower versions. That said, for all practical purposes you do (or you can get them from raid instead). Even the highest level end of dungeon gear needs Wyrm Crests for a level or two.