In Metamask, open Settings->Network->Add Network
Network Name: Arbitrum
New RPC URL: https://arb1.arbitrum.io/rpc
Chain ID: 42161
Currency Symbol: ETH
Block Explorer URL: https://arbiscan.io/
After adding, switch the network to Arbitrum.
You can access C-Chain through MetaMask, by defining a custom network. Go to MetaMask, log in, click the network dropdown, and select 'Custom RPC'. Data for Avalanche is as follows.