Unified memory query

The new iMacs( 24inchers) have this Unified Memory with the M1 chip. Previously iMacs used a portion of the available memory for the graphics card, as I understood it to be. With this unified memory is that not the same?
So if I buy an iMac with 8GB RAM - how much of that or if any is used for the graphics card?
Thanks for the responses.

Howard Oakley has a nice article that explains some of this.