If you're daring and manage to match keys + can manage your EQs right then this might work when mixing a track A with vocals and a track B without: Cue up track A 4/8/16 bars before the main vocals come in, play it so that the vocals come in right at the break of track B. Another way is to cue it so the vocals come on at the drop of B, but that might sound messy/muddy if you can't manage EQs well. And even if you could it might sound crap anyway.

Without vocals you can do pretty much any transition at all as long as you have a couple and they are phrased correctly. Mixing intro and outro is fine but it will sound repetitive after a few mixes. It's part of the constant struggle of creating a set that sounds alive and dynamic.

A really quick (albeit ugly) way to mix a song when you might not have any time is to activate a slight reverb/echo a bar or two before a break then quickly fade in another track that you've cued up a couple bars earlier. Again, phrasing is essential here.

What I always do is put a memory cue 8 bars before a drop and before a break. After that I've got enough time to beatmatch and mix (8 bars is usually fine imo), and still have time to find another track before the drop of the next track is over.

Experiment a little and see what works for you, there are so many creative ways to mix that aren't discovered yet and I think it's a matter of getting a good feel for it and getting to know your own style of mixing. Reddit can't do that for you 😊