#2
Depends on genre there can be a kinda generic strum for certain say country songs but usually it's different and fits the feel of the song.

Really listen to the songs and how the pattern goes. Say to yourself in your head.

down, down, down, up, down, up

down, down, down, up, down, up

Depending on the song of course. Maybe the song is different has pauses maybe it doesn't. Music is a lot about timing so try to focus on it.

It really depends on what your playing.
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#3
I don't think experienced people think much about it. But that doesn't mean it's not important. Like UFC says, it depends on the style of music. Some is pretty much down stokes, some alternating, and a couple use backbeat upstrokes. Beginners probably do well with songs that do alternating strokes, but eventually, you'll find that you either mute one of the strokes (using muted/muffled strings) or your pick just skips the stroke (it moves across the strings without touching). Eventually, you'll build the ability to automatically know when to skip or muffle the strings. That will take time to be able to do that naturally though.