What does the bridge do?
The Rainbow bridge transfers assets between Ethereum, NEAR and Aurora networks. The bridge is fully decentralized & permissionless. It relies only on confirmation on the NEAR and Ethereum networks, without validators for the bridge separately. The time and cost depend on the network conditions of each.
For more info, check https://www.veradiverdict.com/p/rainbow-bridge. As the article above mentions, "Most existing cross-chain bridges rely on centralized validation schemes, like Proof-of-Authority (PoA), where known, authorized validators are responsible for verifying the state of the underlying blockchains". So these bridges are different in the way that they rely on a set of external validators to verify transactions. They are not decentralized in the way that the Rainbow bridge is.
Note that the bridge does not convert tokens. The whole point of the bridge is to allow you to use assets on different blockchains, i.e. the bridge creates a digital token certificate on another blockchain from the original blockchain.
The bridge between Ethereum and Aurora is an extension of the bridge between Ethereum and NEAR. Both use the Rainbow Bridge.
To learn more about the bridge, you can read the following articles:
https://near.org/blog/eth-near-rainbow-bridge ― ample description of the bridge back-end
https://aurora.dev/blog/rainbow-bridge-2-0-design-details ― ample description of the bridge front-end