It is important when purchasing usdt.Because for different reasons, you can use different methods and for different blockchains too. Because we know usdt is in all blockchain networks and then ethereum-based usdt is the most popular, then bsc usdt, and trontron based usdt too. But ethereum network based usdt charges $100+ jusr for transactions on decentralized exchanges. If you need ethereum based usdt, then I suggest you use a centralized exchange where you can use binance and ftx exchange. Then from binance or ftx, you can send it anywhere. because they charge fewer fees. But if you want to use another blockchain-based currency, such as bsc or tron, you can use Binance and Ftx, which also support the max blockchain network for withdrawing and depositing.
 
Or you can use a decentralized wallet/exchange like this: For ethereum-based metamask swaps, uniswap. And for bsc, you can choose between pancakes and metamask swap. And for tron, you can choose to justswap.