Flying patterns

Where on the Web can I learn more about flying traffic patterns? I
see constant references to them in various sources, but nothing that
explains exactly when they are used, and how.

For example, after reading for a while, I think I now understand what
the crosswind, base, final, and downwind legs are. But I can't find
out how long these legs are supposed to be, or what radius or type of
turns are expected to move from leg to leg. Are these patterns of a
standard size, or do they vary from one airport to another? If all
VFR aircraft (or IFR aircraft too?) enter the pattern, and all at the
same altitude, how do they avoid collisions, especially with the
disparate speeds that one sees from one aircraft to another?

I trying flying some of these patterns in the sim this morning. I
didn't do too badly, but I think I was still off the mark. And I
don't know how tightly the pattern must be adhered to.

Does everyone enter the pattern no matter what the direction from
which they approach the airport? Does it always have to be an entry
at a certain spot from a certain angle? I'm confused about how these
work. Pointers to useful Web explanations would be most appreciated.

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