Some questions about Arbitrum and Optimism

Are Arbitrum and Optimism true layer 2's built on top of Ethereum, or are they sidechains like Polygon that are being mislabeled as layer 2?

Why is it that Optimism and Arbitrum can be added to Metamask as separate networks while Immutable X and Loopring users simply have to connect to Ethereum mainnet?

What are the gas fees on Arbitrum and Optimism currently like when transacting with them on NFT marketplaces? Cents, fraction of a cent, 1-2 dollars, 5+ dollars?

What bridge service do most people use to move Polygon & BSC assets to —> Arbitrum & Optimism?

Is there a bridge service that can move from Immutable X & Loopring to —> Arbitrum & Optimism?