You're right, Bitcoin has its own blockchain, Litecoin has another, Dogecoin has another, etc.
To make an easy differentiation, Bitcoin, Litecoin, Ethereum, Avalanche, Polygon are cryptocurrencies because they have their own blockchains, whereas USDT, USDC, DAI are tokens inside another blockchain. So, cryptocurrencies are not tokens as only the former have their own blockchain.
There are bridges where you can bring for example︀ Bitcoin into another blockchain such as Ethereum. Once you use a bridge, you're given a︁ "wrapped" token like WBTC (Wrapped BTC) which is a token in the Ethereum blockchain and︂ is pegged to 1 BTC.
1 BTC (on Bitcoin blockchain) -> Bridge -> 1 WBTC︃ (on Ethereum blockchain)
1 WBTC (on Ethereum blockchain) -> Bridge -> 1 BTC (on Bitcoin︄ blockchain)
Also, forgot to mention, that when you use a blockchain, you always pay fees︅ in the blockchains cryptocurrency which can be Ethereum, Bitcoin, Dogecoin, etc. You will never pay︆ USDC/USDT/DAI/etc to pay for fees. Exchanges usually charge you fees in the same token/crypto you're︇ trying to withdraw to make it easier and charge you higher fees.
If you want︈ to know anything else let me know!