Tokenised BTCs are a way to use BTC on the Etherum (and other) blockchain. tBTC is one of them and other alternatives exist such as wBTC or renBTC.
This websites allows you to:
This article does an interesting comparaison of RenBTC and tBTC.
For the first time, on the 17th of September 2020, Bitcoin Supply on Ethereum has gone above the $1B mark.