As some blockchain projects have migrated from the Ethereum mainnet network to their own blockchain network, compatible blockchain networks can be added as Custom RPC networks, so you can connect to them via MetaMask using your existing account.
Please check https://chainlist.org/ where you can automatically detect the custom network needed and connect it to your wallet.
The following are instructions for establishing these connections manually:
MetaMask Browser Extension:
1) Click on the Account button:
2) Go to Settings:
3) Click on Networks:
4) click on "Add Network":
5) Fill-in the details then Save:
An example of how it should look with all details filed-in:
After performing the above steps, you will be able to see the Custom RPC network in your Networks section.
MetaMask Mobile App:
1) Tap on the hamburger icon in the top-left:
2) Select Settings :
3) Select Networks:
4) Tap on "Add Network" :
5) Fill-in the Custom RPC Network details:
After performing the above steps, you will be able to see the Custom RPC network in your Networks section.