Is there some sort of app or website where I can do creative writing ?

I heard of and signed up to AO3 (https://archiveofourown.org/) but it looks predominantly for smut fan fiction so idk if that's useful.

I’m thinking of like something that has a weekly prompt or something & people can read/ comment on your posts?

Look at comms on !411@literature.cafe (https://literature.cafe/) where there's a creative writing comm, and a Writing Prompts comm, and some others.

My first thought was writing opinion pieces for new sites. Just make shit up seems to be the main strategy.

But maybe you're looking for something more like the reddit group. You could start something here for the same thing.

https://www.reddit.com/r/WritingPrompts/

There is !nanowrimo2@feddit.online that was prompting people to journal so not exactly what you are after but the trackbear app that was used is handy for tracking progress and maybe there’s other communities similar for creative writing.

Linkedin - its all made up but they could use something creative.

I read a lot on RoyalRoad, so maybe try their writing prompts?

There's an app on f-droid called hammer that's basically an open source version of world anvil

It's UI is outdated, nice way of saying it's fucked, but writing.com has a thriving community. I even won a short story award on there once.

Mastodon has a lot of writing prompt posts, especially if you like poetry and shorter bits of writing.