In actuality, this depends entirely on the motherboard manufacturer and their Bios/Uefi team. While the automatic setting is supposed to handle allocations dynamically, and can apportion seemingly up to 1/4 of system ram if left alone on a properly configured system. If AUTO fails to work right, and you have a good bios, you can usually find a setting called "Uma buffer size" which can statically assign a certain amount to VRAM (also seems to be limited to 1/4 total ram). It's usually under Advanced or NB configuration. There are a few videos online of people with 64GB ram setting 16GB and having it available in programs like Photoshop.
I've personally seen 4GB myself on a 4700u.
The "auto" setting is the one that depends on the motherboard vendor and the only thing it does is to set the fixed VRAM.
For what i saw on consumer boards it is up to 2GB with 4G decoding off and up to 50% of ram with 4G decoding on.
But that changes nothing of the dynamic vram, that one is set by the driver. Ill need to check again because maybe they changed it in the last few months, but the dynamic vram was limited to just 2GB last time i checked. If i have time ill check again today.
Normally what do you want is to set a very low amount of fixed VRAM... 64-128MB is just fine. and let the games run on the dynamic vram. But 2GB is just not enoght these days, even for a IGP. Remember that W10-11 uses a non-trivial amount of vram too.
Also remember that fixed and dynamic are two diferent memory partitions, for example, an ETH miner cant start on 4GB Fixed + 2GB Dynamic even trought the miner can see the 6GB avalible, it cant allocate more than 4GB of vram in one go.
That something i noticed some games tend to want to run only on fixed or only on dynamic.
What is very funny about all this is that both implementation of Navi 24 ( RX 6500XT and RX 6400 ) seem to come with just 4GB VRAM... the RX 6400 specially is equal to the RMB IGP except for the dedicated vram and the 16mb ic.
We may come to some cases were the RMB igp is just faster because it can have more vram.