I guess they are the ones responsible for handling our transactions which somehow explains the high TX fees.
Miners confirm BTC on chain tx's, if you are using a custodial wallet, you would pay a fee to the exchange or custodial service to process your tx, what they then do is combine withdrawal requests of many customers and send it in one on chain tx for miners to add it into their block and confirm it with other tx's. BitPay is a payment Gateway, and as hosseinimr93 correctly said, their wallet is self custodial, though i would not call it recommended, because there are so many better BTC wallets that you can use.
I see- this is actually insightful to know. As someone who is really new to this kind of "more secured space" practice, can you recommend me a self-custodial wallet for my privacy? I would very much want to have full control over all the assets, keys, and funds of my BTCs for future transactions.
Use recommended wallets like Electrum, Sparrow or BlueWallet.