Does rootstock use bitcoins or do they use their own rootstock coin? Learning about rootstock. Do they ues bitcoin or their own rootstock coin? In ethereum, they have ether, but to execute contracts it requires gas. Is there an equivalent here in rootstock or all of this is in BTC? What would be the fees then?
Rootstock uses smart bitcoins (SBTC). On a technical level this is a separate token from bitcoin (BTC). They live on a separate blockchain. On the other hand, every smart bitcoin represents one bitcoin.

Rootstock is a Bitcoin sidechain.

This works as a 2-way peg between the bitcoin blockchain and the rootstock blockchain. You use this to transfer coins between the chains. Send a bitcoin over to rootstock and it becomes a smart bitcoin. Send it back and you have a (normal) bitcoin again.

A more technical way to see it is that X amount of smart bitcoins are created by locking the same amount of bitcoins on the bitcoin blockchain. When you later unlock an amount of bitcoins, the same amount of smart bitcoins disappear again from the roostock blockchain.

The important part is that your bitcoin can only be active on one of the blockchains at any given time.