Not all you can do with Lua you can do with a mod, and vice versa.
It takes a while depending on how much you want/need to learn, how much you know about programming, etc.
Lua scripting is client-side, but you can make some stuff usable online, such as create your own HUD or make a bot or a move saver or something.
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Why are there 7 static items in the rotation matrices?
x x x 0
x x x 0
x x x 0
0 0 0 1
The xs change, but those zeroes and ones are always zeroes and ones.
?