firstjois wrote:

So what's the rule here, is it necessary to sand between coats with every
varnish/poly/waterbased poly/oil based poly? Or are they methods of
application where sanding is not recommended?

Read the can. Most polyurethane varnishes have a "window" where they
can recoated without sanding. If you recoat before the window ends, no
scuffing is necessary, otherwise it is.

When wiping on poly, I usually try to stay inside the no-sand time,
except for the last coat. I'll let the second to last coat dry
thoroughly, scuff sand with 320 or 400 grit to remove all the dust nibs,
and then carefully wipe on the final coat.

Other varnishes, like Waterlox Original, Pratt & Lambert, etc... give
better results for me if I sand after each coat, but I often apply
heavier early coats of these with a foam pad. Final coats are wiped,
sometimes slightly thinned.

I've found that sticking with gloss varnish until the final coat or two
prevents flattening agents from clouding the finish.

I don't do much with waterbased varnishes, yet. Others here have lots
of experience with them, I'm sure you'll hear from some.

Practice on scrap!

Have fun,
Barry