How are posts stored in nostr?

For me to keep historical posts available on my profile, will my client need to resend them?

My nostter client doesn't seem to store them locally and I don't expect a "relay" to store them for long.

I've been trying to replace Reddit with Lemmy, but especially for those large communities like /r/DataHoarder the smaller, federated communities seem to lack quantity and quality of interesting posts.

I'm now looking at the dependencies of the nostter client. It uses nostr-tools and rx-nostr from NPM.

Where to find up-to-date nostr SDKs? Ideally, I would like to build a new client in TypeScript.