Does the phone show any signs of life at all, like buzzing or vibrating when you plug in a charger or press the power button, or else being recognized when you plug it into a computer?
You could be looking at a dead display, a dead battery or both. Those are the two parts most commonly damaged by liquid exposure. If you've got signs of life, I'd start with replacing the screen first, but if not the battery would be the place to begin.
The battery isn't too bad to replace, but the screen can be a bit of a pain if you need a new one. Here are guides for both repairs in case you need them.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-a21s-1238/bater-a-11032|Manuales / Samsung Galaxy A21s / Battery]
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-a21s-1238/pantalla-completa-11033|Manuales / Samsung Galaxy A21s / Fullscreen]