They cost a shit ton though because they have to run on mac mini AFAIK. It would most likely be way cheaper if there were to run on commodity hardware.
Yes for renting an instance from AWS. Minimum lease time is 24H iirc. But that's not the case for Github Actions (and so I assume Azure Pipelines). You pay per minute, it's just 10x the cost of an Ubuntu instance.
Azure Pipelines supports macOS 11 and 12.
GitHub Actions supports macOS 11, 12, and 13 beta.