Is Axelar a blockchain and how does it work?

Axlear is a cross-chain interoperability network. It is not a L1/L2 blockchain but instead Axelar runs on Cosmos SDK using Tendermint consensus algorithms (meaning Axelar is built on top of Cosmos).

For a quick introduction you can read through here How Axelar Works. Included in the article is also a video that breaks down how Axelar functions.