Bitcoin vs USDT is a comparison between two different jobs. Bitcoin has no issuer; new coins come from the network's own rules, and its price is set by buyers and sellers, so it can rise or fall sharply within a day. USDT is a stablecoin issued by a company that aims to keep each token equal to one unit of a traditional currency, backed by reserves it reports holding. Its steadiness rests on trust in that issuer, which can also freeze particular addresses. Neither is better in general; each suits a different need.
Is bitcoin the same as cryptocurrency? No: bitcoin is one cryptocurrency, the first and best known, while the word covers thousands of coins and tokens built on similar ideas. Some, like bitcoin, have no central issuer; others, like USDT, are issued and managed by a company even though they live on public blockchains. Treating every coin as a copy of bitcoin hides real differences in who controls the supply, what backs the value and how the network charges for transfers. Comparing bitcoin with any other coin starts with those three points.