There are two traditional, exchanged traded futures contracts.
-The ICE Bakkt (BAKKT)contract is for 1 bitcoin and is physically settled (meaning the holder at expiration is entitled to receive 1 “cash” bitcoin, retail traders are usually not allowed to hold contracts to expiration and take delivery though).
-The CME BTC (BRR) contract is for 5 bitcoin and is cash settled.
As with other futures contracts, selling short is just as easy as going long and instead of putting up the notional trade value you put up a margin instead. The margins for both contracts are higher than most other futures and some brokers require additional margin on top for a short sided trade. Last I looked the overnight margin for the BAKKT contract was about 1/3 the price. As long as you have enough free capital in your acct to cover the margin, you can hold the position. This is what gives futures leverage.