Polygon Mumbai, Avalanche Fuji
Once you've connected your Metamask to Goerli, connecting to other EVM testnets such as Mumbai and Fuji is very simple. Go to https://chainlist.org/ In the top right, connect your wallet and make sure to tick the "Include Testnets" box.
After doing so, simply look up the name of the network you wish to add and click "Add to Metamask"
Approve the prompt in your metamask wallet:
And that's it! you are now connected to the relevant testnet.