how does an operating system say linux understand how many ram/pci/cpu slots/sockets are there?

Does the operating system detect this or just the info is passed from bios acpi if so please tell how even bios detects this?

Modern OSes scan the hardware again as much as i know but i am confused when it comes to slots.

I understand how they can detect the device installed in it, like for instance the pci devices are detected by bus enumeration attempting to read vendor ID and Device ID for each combination of bus number and device number.

How are ram/pci slots and cpu sockets detected? like how many are there.

Thanks for the help

This may help: https://appuals.com/check-ram-slots-linux/