Since I can’t add links - see this post where Luigi shares how to add BSC network to MetaMask. The OP is writing in spanish but you can follow what Luigi shares by his pictures. By default, MetaMask loads initially on Ethereum Mainnet. You will have to add BSC.
When you’re doing any transferring just be sure you’re choosing the right tokens/addresses for the same networks. You can’t transfer to different networks without bridging.