macOS Sierra Version 10.12.6 to MacOS Big Sur

• How to get older versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

• How to create a bootable installer for macOS - Apple Support

https://support.apple.com/en-us/HT201372

Both of these site pages offer ideas to use an aging/older Mac.

Big Sur is not compatible with your model MacBook. Here is the list of devices that are compatible. If your device was compatible, you would go to Software Update in your settings and be able to download it there. When you go there if it shows that your computer is up to date, then your computer's hardware does not support any version higher than what your computer already has installed on it. At almost 10 years old, the hardware in the 2012 MacBook just is not able to support the OS and the features that are built into it.

According to this article, MacBook Pro 2012 model is not compatible.

Ref: macOS Big Sur is compatible with these computers