Hi albert,
Well, unfortunately I can't offer you a lot of hope here, as salt water is very bad for electronics. First off, ***do not try to turn the phone on or charge it until it has been cleaned!***
About the best you can do at this point is to either take it to a repair facility that is experienced in liquid damage for a thorough cleaning and repair, or you can see about doing that cleaning yourself. You'll need to be comfortable completely disassembling your phone, cleaning it and putting it back together. Here's a Wiki on what to do when your electronic device gets wet.
[[Electronics Water Damage|Electronics Water Damage - iFixit|new_window=true]]
You'll need to take the phone apart and clean everything. Here's a teardown for your phone that should give you what you need to know.
[link|https://www.youtube.com/watch?v=j5qZ4SgUOB4|Xiaomi Redmi Note 9S Teardown - YouTube|new_window=true]
One final note: ***Buy a new battery!*** Salt water is extremely conductive and has probably destroyed the battery, and if it hasn't already, it will.