Still trying to dry fit the ABS under my sink before I glue it.
Is there something I can do to make it easier to fully seat the ABS street elbow all the way into the cup of the fitting? It gets stuck about half way and doesn't want to go all the way to the depth of the fitting's cup. Same with the trap adapter going into the tee-wye.

the male end of street fitting and pipe itself are both nice and straight. (no taper)
the socket of a fitting (or what you call a cup) has a taper to it. it is actually smaller diameter the further you go into it. if you get your fitting half way into the 'cup' that is about right. have a little confidence in your measuring skills and glue it together. it will go all the way in once the glue softens and lubricates it.

OK ... good to know. I've never worked with this glue before and I am a little skittish about it ... so it will reach the depth of the fitting once the glue is applied and I give it a twist? OK.

after you apply the glue, let it set for about 10 seconds before inserting the fitting and let the plastic soften just a bit. That gives you time to apply the glue, and carefully put the applicator back into the can, and possibly move the can so you don't inadvertently knock it over. There is nothing worse than knocking over a can of glue when you are in a hurry, except for knocking over a can of purple primer. Do that and you can count on replacing a floor.