That depends on the VPN service you want to use. But most of the time, the process will start with plain data you want to share. Then the VPN will re-arrange the data and apply a key to transform the data back to its initial format. Now the data is fully encrypted and only the VPN service will know how to unlock it with the dedicated encryption key. There are multiple encryption protocols that the VPN service can use, with the AES-256 encryption system being the most popular one.