What I did:

Read how DNS and IP work``
What is ICANN
ICANN is bulk seller, whereas godaddy, Namecheap, bluehost etc are retailers

browser-> goes to registrar (godaddy etc)-> goes to ICANN

NXDOMAIN --> non-existent domain (what browser themselves shows when IP not found on DNS/ website not found)

how mail look like in server. You can make your own mail service from scratch just like team shiksha has its own

Like Microsoft teams , we can see chatbox section anytime, and read chat. it because it has a chat server, similar to it we have mail server for mail services

mail exchange like mail server(consider website server which serve website but mail server serve mail)

If you have your own mail exchange server and your friend has his own mail exchange server, you both will be talking privately and nobody will able to track it ever.