I have made myself a simple home in Riverwood. I am not an expert with the Creation Kit; but how can I make it so that companions will follow me into this interior cell? They just don't seem to enter at all.
Is there some kind of marker I need?
Is there some kind of script?

There is one more thing, how can I make this home / interior cell I have created as a possible place for the ones I marry to move in?

Did you include the navmesh?

No, no Xmarkers, no Northmarkers nor any Navmesh...
Are these markers necessary for this?

Navmesh is required for NPCs to navigate in your home. There's a basic CK tutorial. The easiest way to get your spouse in is an excellent mod by Amgepo and Emma called Spouses Can Live Everywhere.

I'd like to add it's very important to finalize the Navmesh, too. When you do, you'll see the triangle closest to the front door will turn green. The AI uses that triangle to know that that's a transition point between the indoor and outdoor cells. And ditto on Amgepo and Emma's mod. So far, it's the best solution to allowing spouses to move between multiple custom houses (and they're planning to add functions for other companions, too).

Thanks for the help.

So many helpful people, thank you and all.

There's a lot of good videos worth studying here that can answer a lot of questions. It's a sticky on the first page.