I think both! There are parts of mathematics that just look like truths that have been waiting the whole time to be discovered. On the other hand, people invent

I think both! There are parts of mathematics that just look like truths that have been waiting the whole time to be discovered. On the other hand, people invent problem solving techniques which definitely feel more like inventions than discoveries. Then in the middle, there are made-up mathematical structures introduced to bridge between two “clearly discovered” canonical objects, but this made-up structure certainly has the invented flavour. So I think it is a continuum, and really fantastic mathematics will feature ideas from all the way along the spectrum: “discoveries” for the beauty, “inventions” for the problem solving, and the “in between” for the subtlety and art.