Are Steam games kind of sandboxed?

Hi all, I am trying to access a folder directly beneath the root folder of my computer from inside a game (Symphony), using Steam Linux Runtime (no Windows emulation like Proton). Strangely that particular folder is missing (some others like /boot and /opt too). Then there is a folder /overrides that does not exist in /. So this is some kind of sandbox, but I have no idea where to look, if I want to whitelist or link other folders there. Can anybody help? BR Markus