The main Free TON network is main.ton.dev
The main network is available in Surf. You can purchase Crystals in Surf: Surf main page -> Top up.
Devnet runs the latest stable version of the Free TON Blockchain. The goal of the devnet is to provide a stable development environment for application developers. It is optimized to connect with TON Labs SDK. Read more here: https://docs.ton.dev
You can easily switch between the networks in Surf: Settings → Advanced settings → Developer mode.
Please note that your Surf address is the same for mainnet and the devnet; however, the address for mainnet cannot be deployed while your TON Crystals balance is zero.