One point to note is that, for some questions, it is possible to do experiments to get data. Certain questions are things we now have computer programs to generate, and previously they could have been done on a far more limited scale by hand. So in some cases mathematicians do work more like experimental scientists. On the other hand, once they've found what seems to be a pattern, they change approach. Gathering further examples isn't much use (unless you then find a counter-example, but it can be encouraging) - you need to find an actual proof.

More generally, nearly every big result will come from some 'experiments': you try special cases, cases with more hypotheses, extreme cases that might result in failures...

On the 'copy-and-paste' point, mathematicians do use a lot of what other people have done (generally they must), but whereas you might copy someone's code to use it, when you cite a theorem you don't need to copy out the proof. So in terms of written space in a paper, the 'copied' section is very small. There are (fairly large) exceptions to this: fairly often a proof someone has given is very close to what you need, but not quite good enough, because you want to use it for something different to what they did. So you may end up writing out something very similar, but with your own subtle tweaks. I guess you could see this as like adjusting someone else's machine (we call things machines too, but here I mean a physical one). The difference is that generally in order to do this sort of thing you must completely understand what the machine does. Another big reason for 'copying' is that you may need (for actual theoretical reasons or for expositional ones) to build on the actual workings of the machine, not just on the output it gives.

More to the point of the question: As a mathematician, you generally read, and aim to understand, what other people have done. That gives you a bank of tools you can use - results (which you may or may or may not be completely able to prove yourself), and methods that have worked in the past. You build up an idea of things that tend to work, and how to adapt things slightly to work in similar situations. You do a fair amount of trial and error - you try something, but realise you get stuck at some point. Then you try and understand why you are stuck, and if there's a way round. You try proving the opposite to what you want, and see where you get stuck (or don't!).

Once you have a working proof, you see whether there are closely related things you can/can't prove. What happens if you remove/change a hypothesis? Also, does the reverse statement hold? If not entirely, are there some cases in which it does? Can you give examples to show your result is as good as possible? Can you combine it with other things you know about?

Another source of questions is what other people are interested in. Sometimes you know how to do something they want doing, but you didn't think of it until they asked.

One more point I'd like to make in the 'methods of proof category' is that, for me at least, there's a degree to which I work by 'feel'. You know those puzzles where all the pieces seem to be jammed in place but you're meant to take them apart (and put them back together again)? You sort of play around until you feel a bit that's looser than the rest, right? Sometimes proofs are a bit like that. When you understand something well, you can 'feel' where things are wedged tight and where they are looser.

Sometimes you also hope that lightning (inspiration) will strike. Occasionally it does.

(All of this may not exactly answer the question, but hopefully it gives some insight.)