What happens when a Defi goes offline?

I deposit and borrow from Aave and was wondering what if the site goes down? By decentralised, does it mean the whole site (eg: aave.com) is run on decentralised servers or does it mean they are just a ethereum/polygon/blockchain smart contract and the site only serves the UI for the user?

If it's the latter, if the site goes down, where do I go to withdraw my tokens? Do I have go through their smart contract and manually interact with it by writing code? Or will I be able to fork their code and at least get my own version of the UI?