Wallets display bitcoin in BTC, but the protocol itself thinks in satoshis: every balance, fee and output is an integer number of sats. The subdivision is fixed by consensus — there is no smaller on-chain unit, and no way to create one without changing the rules for everyone.
Sats make small amounts readable (5,000 sats beats 0.00005 BTC) and they are how fees are priced, via sat/vB. Convert any amount with our live converter — and don't confuse the unit with the unrelated "SATS" token that borrowed its name.