But beyond that simple progression, there are more details and nuance to consider. Let’s say you have completed the game on Normal difficulty, and gathered gear as you went - the game’s default expectation is that you should now be ready to begin playing on Nightmare difficulty, and that by the time you clear it on Nightmare, you will be geared enough to begin on Hell difficulty. However, once you’re at a certain gear level you can kick the difficulty of the game up a notch. You kill things, they drop gear, sometimes it’s an upgrade and you keep it.
Essentially, you gear up by playing the game - there are no Real Money Auction Houses or microtransactions here. Gearing up in Diablo 2: Resurrected is at once much easier in concept than in more modern ARPGs, while also being somewhat complicated in terms of discussing the finer points.