1 BTC equals 100,000,000 (one hundred million) satoshis. The satoshi is the smallest unit; one cannot transfer less than 1 satoshi on the Bitcoin network.
A bit (also written μBTC, or microbitcoin) equals 100 satoshis, or 0.000001 BTC. It was popularized as a more human-friendly unit at small denominations but is rarely used by modern wallets.
mBTC means millibitcoin — 0.001 BTC, or 100,000 satoshis. Used occasionally by exchanges and Lightning wallets for medium-size amounts.
JavaScript's Number cannot represent integers above 2^53 without precision loss. Total bitcoin supply is 2.1 × 10^15 satoshis — well within range — but multi-amount math can still drift. BigInt keeps every conversion exact.