Is Polygon / Matic an L2 or a sidechain?

Hopefully someone familiar with Polygon / Matic network can answer the following for me, as I've not been able to find it.

Can a user unilaterally withdraw from Polygon / Matic network using only the information published on Ethereum?

How does it approach the data availability problem? Is the data available on Ethereum L1?

I ask this because an article I read which described a sidechain vs an L2 mentioned this difference and I was curious if Polygon / Matic is then considered a true L2 or just another sidechain?