Lizzy here, from Shopify.
When you first create your Shopify account, the PayPal gateway on your store will be set up using the email address that you signed up for Shopify with! If you don't have a PayPal account connected to that email address, you will have received an email at that address when your first PayPal order came in. That email will contain instructions for how to start depositing the funds from your orders into PayPal.
If you run into any trouble finding the email I mentioned above, or depositing the funds, I recommend reaching out to PayPal directly. You can use the information in the Order Timeline to talk with their support and locate the funds in their system. That spot will look like this :
If you decide to deactivate the PayPal Express gateway the money for those orders will not be affected. That being said you will want to make sure that you complete your account set up, as PayPal will refund the money to your customers if it is not sent to an active PayPal account within 30 days from when the order was placed.
If you do remove and re-install the gateway, any subsequent orders placed using PayPal will be sent to the account that you connect upon reactivating. All previous orders won't be changed.
Please don't hesitate to let me know if you have any further questions about this!