You cannot upgrade soldered RAM on a Chromebook.

No. It is physically attached to the motherboard. There is no slot, no socket, no upgrade path. What you buy is what you get, forever.

Chromebook makers solder the RAM to save space and cost. That means zero upgrade options after purchase. Some higher-end models use removable RAM, but those are rare. If you are looking at a sub-$300 Chromebook, assume it is soldered.

As for software limitations: Chrome OS does not impose extra limits beyond what the hardware supports. But that does not matter because you cannot physically add more RAM. The BIOS cannot help you. No amount of software trickery will unsolder those chips.

Buy the model with enough RAM from the start. 8GB minimum. Do not cheap out here.