OP, moisturizing and sealing is done to the hair. You apply some water-based (or as I'm discovering from moisturizing my skin aloe-based) product to your hair to soften it, and then you apply something heavy especially to ends to seal.
I am not a product lover so one way I think of doing this (if ever I do) is the way a member Msa used to do it. She'd moisturize her hair with water, and seal the entire length with Castor oil.
Back when I wasn't yet a member, I would apply a silicone serum to my damp hair and it seemed to seal my hair from losing moisture I got from washing/conditioning.
Also some people do seal their ends with something heavy like Vaseline.
The reason people focus on ends is because they are the oldest part of the hair and the part most likely to have the most damage from being around longest, and to break off and thwart your progress. It wouldn't hurt to seal the entire length, but if you don't want to do that, then at least focus on your ends.
But what is even better than just sealing is hiding your ends away (protective styling) so that they are safe from the elements (air, sun, friction)...which wreck havoc on ends and affect retention.