As with all recurring services in a WHMCS installation, WHMCS will automatically invoice your customers for the renewal of their services as they approach the renewal date.

Only if and when your customer pays the renewal invoice will the service be renewed via WHMCS MarketConnect.

Any MarketConnect services for which the renewal invoice is not paid will expire.

For the avoidance of doubt, services will never auto renew with WHMCS MarketConnect.  Only an admin triggering a renewal action manually or a customer paying an invoice for the renewal of a service can trigger a renewal action and charge to you.