What goes between coats of oils of a work in progress?

The rule "Fat over lean" applies.......

you don't have to re-coat oil paint because it takes months to dry fully (without the use of a drying agent.) It may feel dry but it really isn't until it's been sitting for a long time. So, you just treat it like you normally would, remembering to be fat over lean (use more galkyd/oil in top layers than you would in bottom layers to prevent cracking)

Some artists use a light coat of retouch varnish when they go back into a painting that has been sitting for a few days. It brings up the surface sheen of the drying paint to more closely match that of the new paint being applied. As far as I know this is strictly for the purpose of being able to better visually judge how well the new paint is matching the old.

I'll do that if it's been sitting for a couple months and is no longer as absorbent.

Note: it is important that you use RETOUCH VARNISH (as mentioned) because you can't paint over normal varnish.