The loop is simpler than it looks
You start a run on a stage and your character moves on their own. Enemies spawn in waves and walk toward you. Your weapon fires by itself.
Your only real job is the choices between waves. That is where the game lives.
Gems are your experience
Every enemy you kill drops gems. Walk over them to collect. Fill the bar and you level up.
A level-up opens a small menu. You pick one option: a new weapon, an upgrade to a weapon you own, or a passive item. Each pick changes how the rest of the run plays.
The 30-minute wall
Most stages run for about 30 minutes. Near the end a powerful boss appears. If your build cannot kill it fast enough, the run ends there.
This is the damage check people talk about. The fix is not luck. It is having picked weapons that scale, not weapons that looked strong at minute five.
What the run teaches you
The first run feels chaotic. After a few, you learn which picks compound and which stall. The best early game build shows a path that compounds from the start.