Celestia is the first ever modulair blockchain.
Celestia is build in two layers, a consensus layer and a DA layer.
The Consensus is build as a tendermint chain.
The actual use case for celestia is on their DA layer.
On the DA layer you can build an rollup. Where it is not needed to have a consensus layer.
In this matter it will use the consensus from the celestia tendermint chain.
At the moment there are a couple off testnets running.
Mocha – Long lasting testnet
Arabica – Devnet
BSR – Incentivised testnet
Please also join their discord to ask questions if you have them.