Polygon PoS it the official bridge provided by the Polygon team to bridge assets from Ethereum to Polygon chain. The bridge is validated by Polygon validators and allows for asset as well as data movement between Polygon and Ethereum.
This is a very typical Token Bridge that locks tokens in the escrow contracts on Ethereum and mints tokens on the Polygon network. When bridging back to Ethereum tokens are burned on Polygon and then released from the escrow on Ethereum.
Note: This section requires more research and might not present accurate information.
Validators on the Polygon network watch for events on Ethereum and when they see that tokens have been locked they mint new tokens on Polygon. Around every 30 minutes validators submit new Polygon state checkpoints to the Ethereum smart contracts. To withdraw tokens users need to present a merkle proof of a burn event that is verified against the checkpoints.
Users can be censored if validators on Polygon decide to not mint tokens after observing an event on Ethereum (CRITICAL).
Funds can be stolen if validators decide to mint more tokens than there are locked on Ethereum thus preventing some existing holders from being able to bring their funds back to Ethereum (CRITICAL).
Funds can be stolen if validators submit a fraudulent checkpoint allowing themselves to withdraw all locked funds (CRITICAL).