BTC is the ticker for bitcoin. In code, 1 BTC = 100,000,000 satoshis (COIN in amount.h). Supply is capped below 21 million BTC.
Prices are quoted per BTC, but you can buy any fraction.
Takeaway: You don't need a whole BTC; 0.001 BTC is 100,000 sats.
Glossary · deep dive
BTC
The ticker symbol for bitcoin. 1 BTC = 100,000,000 satoshis.