SSL/TLS connection from android to a Custom Socket Server

Finished coding a simple Chat Server in erlang that uses gen_tcp (simple sockets) and an Android Client app for TESTING.

Now i need to implement SSL/TLS.

I kept connection (Sending and Receiving Data) part in a single module, a few lines of code that i can upgrade easily.

I have no prior experience of implementing SSL/TLS so i am confused here (like a lot).

Questions - Testing part (localhost):

I can generate self signed certificate by:

openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365

which results in two files, key.pem and cert.pem, My understanding is that one is Server certificate and the other is the key file but

1. Where is Certificate Authority file? Do i need to generate one? Do i need one?
2. Are they enough to test my app, to make it ready for a release?
3. How do i turn my simple android socket into a secure socket? (kotlin preferred)
4. Can i trust my self signed certificate programmatically (and only that one) or do i need to install it on my device (and any other device like a friend's device)

Questions - Production part:

5. SHOULD i use my self signed certificate on my server? Do i need to create a CA? Pros and Cons of using self signed certificate?

6. Can i use free certificates like from letsencrypt forever? Pros and Cons?

Finally:

1. Do i need a certificate for Client too? Do i need to provide some sort of key[s] to Android app?

2. What can i do to protect my app from MITM (or protect my app from revealing what is being sent/received to and from server) for example THIS TYPE OF ATTACK

from SSL/TLS connection from android to a Custom Socket Server