Before you upgrade to a new OS like macOS Big Sur it is vital you check that any apps that you use with your mac will
continue to work with the new OS. This is especially true because from the introduction of macOS Catalina two
years ago new mac OS's no longer run 32bit apps, and this will continue in future macOS's.
Before upgrading it would have been helpful for you to download and run this free app, https://www.stclairsoft.com/Go64/
This will list all your apps that are 32bit or have 32bit elements, and you can contact the apps developers to see if
they are providing 64 bit updates or alternatives. The same also applies for any printers/ scanners or other hardware you use
with you mac. You can still run the app now, and then check for updates.
Unfortunately when you are on macOS Big Sur it will not let you download older macOS full installers like macOS Mojave.
Can you source another mac that is running an older OS which is capable of downloading the macOS Mojave installer.
On that mac,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on macOS Mojave.
This will redirect you to the App Store Mojave download page,
it should say Get or Download.
Once it has completed downloading.
Make a bootable USB installer using these instructions,
How to create a bootable installer for macOS – Apple Support (UK)
Now make a full back up of your mac.
Insert the bootable USB directly to your mac.
Restart your mac while pressing and holding down the option/ alt key.
In about two minutes you will see the Boot Manager, select the USB and
press Return. The mac will now boot to the USB.
From the Utilities panel select Disk Utility and press Continue.
Click View in the menubar and select Show All Devices.
Highlight the Disk (Apple HDD..... or similar) not any of the indented Volumes.
Click Erase.
Give the Disk a name.
Format: APFS.
Scheme: GUID Partition Map.
Click Erase.
When Done quit Disk Utility.
Click on Install OS and press Continue.
The installation procedure should start follow the prompts.
Once the installation has completed and you have set your mac up
you can then restore only your personal information from the back up
you made prior.
Do not restore any system or library files, this will put Big Sur back on your mac.
Do not restore apps, this will put the Big Sur versions back on your mac which
will not work on Mojave.
Only restore your personal data, but this comes with possible problems too.
Any files you edited or created in Big Sur may not be backwards compatible
with the Mojave versions of apps.
If you opened your Photos library and Music library in Big Sur and upgraded
these libraries to work with Big Sur you cannot downgrade them back to a
version that will work with Mojave.