The CoinPayments adapter sets the fixed portion of the withdrawal fees to be equal to whatever amount is required by the CoinPayments platform to pay for blockchain mining. Any additional fees that you set are added on top of that. The extra fees that you set are subtracted from the user’s account but not from the wallet address.
Please see this recent post to a similar question for more details: https://www.dashed-slug.net/forums/topic/failed-withdraw-and-fees/page/2/#post-2325
The video you are referring to was done before any of the web wallets were implemented, but the general idea still holds. While earning some transaction fees can make you money, you cannot expect to create a website wallet with only that as your plan. Your users need a reason to use your website as a wallet, so you need a business plan. This is where the app extensions come in. They let you provide functionality on top of wallets.
Hope this is more clear.
kind regards