PS: You could also use something like ManyVerse:
It works well, but be warned: it potentially uses a lot of storage and a lot of CPU/battery. And if you use it while your phone has internet, it could potentially eat all of your data plan in no time.
This problem will only be well and truly gone when we have open source and decentralized options that *can't* be abused... and that have familiar enough interfaces with rich enough features to be acceptable to 95% of people -- rather than the 5% hardcore geeks (like me) who use such alternates now.
If you can code, or have a little extra coin, consider contributing to open-source alternatives.
There is some new info on setting up a Room.
https://www.manyver.se/faq/connect-room/
Pub: meet people on a server. you can see their public posts, they can see yours. it's a way to make public conversations, eavesdrop on other public conversations, see things people have posted on their public boards, etc., so to speak. That means you will be downloading other people's data (who are in the Pub) and sharing it around. Often Pubs have a local topic of conversation, "This pub is for people interested in Quilting" or "Exchanging recipes".
Room: is more like a quiet room, like a place where people go to pick up and drop off their mail, including mail they are passing along through the network to friends and friends of friends. You can interact with someone if you want, or not. You can tell a friend who you cannot meet in person (to connect over local wifi) to meet you there so you can both follow each other and make a connection. Mostly mail is distributed through the network. So, if someone is there that you do not follow and none of your friends know them, you're not exchanging any data with them.
As it happens, there is a peer-to-peer social app: Manyverse. It's based on a protocol called scuttlebug and as far as I can tell, it doesn't need a backend at all. It works solely peer-to-peer.
(Diaspora, and more recently mastodon, do not rely on 'a' central server, either - strictly speaking. They use distributed servers and if my understanding of it is correct, there is nothing that controls the network. I would, however, be the first to concede that there is a backend in the classical sense.)
This is not what I had in mind, though. OP asked if he needed a website, or if he could start with just the app. You failed to address that question at all, and what you did say - namely that a website was needed for the backend - is simply not correct. Most social apps do need/use some form of a backend, but that's simply not the same as a website.
Words mean things. Someone who needs to ask of they need a website is unlikely to understand what you meant by your answer. In fact, I didn't understand your answer, either.