None of the notes, images or videos people post are stored on relays, they're not servers. They do cache notes for awhile, but also do purge their caches.

However, there are relay operators that will store your posts permanently on their relay, for a fee.

Or...

You can spin up your own personal relay that runs in your system tray in just a few seconds. If you're an influencer or important person, you may run your own personal beefy relay if you're producing your own video content etc and you want to make it available permanently.

You can even spin up a relay as a cloudflare worker app in less than a minute. A relay is a pretty simple piece of software, it's not doing much other than relaying tiny strings of json text.

Otherwise, if you're a nobody like me, happy to use the default relays my nostr client chooses. I don't care if my posts vanish after a time. It's not like I'll ever say anything profound, or world changing statements needing to be preserved for future generations 🤷. I certainly don't think I'm saying anything worth me even running a small private relay.

There's also a lot of people offering filtered relays that people can use so you can watch the Global feed at work 😜.

Because nostr is a protocol and not a company or service, you have to store anything you want to keep permanently yourself. That has to be better than relying on Twitter a federated server admin to store your data for you. For free. Because that makes you the product they're selling