Great question! If Shopify Payments is enabled, but your customer pays using Paypal, then you would only be charged the Paypal transaction fee. Shopify will not charge you the credit card fee, or transaction fee.
If you choose to disable Shopify Payments, then you would be charged a transaction fee and the fee Paypal charges. More information can be found here: https://help.shopify.com/en/manual/markets/pricing/fees