Initially, your MetaMask account will appear on the main Ethereum network.
Click on the "Ethereum main network" section. Subsequently an "Add network" option will appear.
Once you have clicked on "Add network", the following window will appear, in which you can add the details of the network you wish to add. To do this, you should always make sure to obtain the details from the official sites of the network you want to add (Polygon or Binance, for example), to avoid any possible scam attempts.
Once you have filled in the details of the network you wish to add, click on "Save". After that, MetaMask will notify you that the respective network has been added.
Now, the next time you click on the "Ethereum main network" button, you will be able to select the network you have added and start trading on it.