How is basic availability group able to auto failover without a cluster?

I have recently learnt that the SQL server standard edition's basic availability group doesn't require a windows cluster. Basic AG is considered equivalant to the deprecated mirroring technology. Like mirroring, the basic AG supports auto and manual failover. However, mirroring supported auto failover only with the presense of witness server. In availability groups there is no concept of witness server. So my question is - how is basic AG able to support automatic failover without the windows cluster? Or is the concept that - basic AG without cluster only supports manual failover, and basic AG with cluster supports automatic and manual failover? I couldn't find documentation on this so any guidance will help me here.