Hi and welcome here.
The Fediverse (of which Mastodon is a part of) is decentralized, like e-mail: anyone with a gmail account can write to anyone with an india.com account (I'm not familiar with what email servers are used in India, sorry), or a yahoo account, or any mail account actually.
Anyone can even host one's own mail server and then he'll be able to write to any other mail account.
It's the same with the Fediverse: I am on Miaou.drycat.fr and you are on mstdn.social but we can still write to each other and see each other messages: it's called "federating".
Federating is a choice; many instances chose not to federate to the nazi/alt-right instance named "gab", for example. But your instance federates with mine; if it didn't there is not way we could see each other's messages.
OK, I didn't define what is an instance, so bear with me.
If you want to have your own mail server, you have to host it somewhere: that is, to install on a computer connected to the internet (either your own or one that you the software that can handle sending and receiving mails (which in turns means that the software will follow the email protocols that are standardized). Usually you also gets a domain name (usually from an internet registrar) in order to use a mail adress (like firstname.lastname@example.org) rather than an IP adress (like email@example.com).
Once it's done, you can create accounts for other people: you can have firstname.lastname@example.org and email@example.com and firstname.lastname@example.org. You can even automatise account creation in order to let other people create their own accounts.
That's exactly what gmail or india.com or yahoo did in order to launch their services (well, you can guess that nowadays they have a bit more than one computer to handle the millions of emails they send each day, but the principle is the same).
It's the same with Mastodon or the other softwares that work on the Fediverse: in order to have a working instance, someone has to install the software (be it Mastodon, Pleroma, Peertube, or any one that can work with the Fediverse protocol named ActivityPub) on a computer connected to the internet, either at home or on a rented server.
Once it's done, this installed software will be a new instance: it will be administred by the one who installed it (or the one he transfers control to) in the way he decides to.
Some instances are personal (the guy who administers it is alone on it), some are invite-only (one cannot join without asking first), some are open registrations (anyone can join, like you joined mastdn.social), some are closed registration (they do not accept new users), but they still federate with each other.
Each instance administrator makes its own choices: when he updates the software, what rules he implements on the instance (the funniest example is oulipo.social; on it it's forbidden to use the letter "e"), who he will ban or accept and why, and so on.
One important thing to understand is that the instance's administrator has access to all the private messages of the instance's users : I'm not saying that he actually reads them, but he has the technical ability to do so (no one else can as they are encrypted - to the exception of course of the user you send the private message to, and of his instance's administrator).
Feel free to ask more questions if you so need.
(oh, another example of specific rules of different instances: I believe mastdn.social limits the length of your posts to 500 characters, and you may have noticed that my posts are sometimes longer than that: that's because my instance's administrator chose a limit of 1024 characters instead)
Thank you, appreciate the detailed response.
There's an indian instance I discovered today, https://connect.fshm.in/ if you'd prefer (it's not mastodon, it's pleroma, but basically it's the same and they federate without problem).
Installing your own instance is not so hard (I failed to complete the process because the tutorial I followed was bugged and I never took the time to go back to it) but apparently maintaining it is harder especially if you don't know Ruby (I don't).
Pleroma seems easier to self-host (@email@example.com does and he's not a IT tech) but the administration interface is command-line only.
I wrote (with a few typos alas) about alternatives here: https://miaou.drycat.fr/@LienRag/103092841923010025
Le réseau social de l'avenir : Pas d'annonces, pas de surveillance institutionnelle, conception éthique et décentralisation ! Possédez vos données avec Mastodon !