Compatibility for Mid 2012
What is the newest OS I can install on my Mid 2012 Macbook Pro, since it is not compatible with Big Sur? And how can I access an older OS to install it?
MacBook Pro 15″, macOS 10.13
You can boot into Internet Recovery Mode using Command + Option + R to access the online macOS installer. If macOS 10.12.6+ was installed, then you should be able to access the macOS 10.15 Catalina installer, otherwise the oldest version of macOS will be installed.
https://support.apple.com/kb/HT204904
You can find links to various macOS installers from 10.11 to 10.15 within this Apple article as well as instructions for creating a bootable macOS USB installer:
