Questions About Layer 2 Scaling

I am very confused with all this. Please explain everything to me as if I know nothing. Is this going to be an Ethereum upgrade or a new network launch? I have read some articles about Optimism and as far as I understood it’s a new Network? If it is a new network launch then how can I bridge all the assets when it comes out? If so, not all erc-20 tokens will be available in the new net which means it’s slghtly useless like BSC? Is there a date on when all these will be released? Can someone explain these very simply? I heard multiple different stuff everywhere so I am really confused with all these.