I replaced the screen of my partner's note 10 pro after she destroyed the OLED screen by immersing it in water. When I put...

You haven't given much information, but given what I know about mobile phones in general, unless they have specific protection against water immersion, you're likely out of luck. The Redmi Note 9A has no IP rating as far as I know, so if it fell in the water the electronics inside got wet. If there was any power in the battery the water likely caused a short and some components got fried. Soldering new components requires specialised equipment and lots of skill/experience. You could get them replaced but it'd probably cost more than the phone. If nothing was damaged by an electrical discharge you may still have some corrosion from the water. This could also be repaired, but would also likely cost more than the phone. Your best chance of salvaging anything at all is to dismantle it as much as you are able and allow it to fully dry out. After a week or two you could try charging it and see if it turns on. Don't hold your breath that it works. It's probably toast.