Please correct me if I am wrong. But it seems to me actions like format value or replace in theirs "Fields" boxes need to have the name of the tag field written without being placed between %%
But when it comes to guess value action, not only it denotes the source as "source format" instead of "field" but also works differently- it needs those %% or else it will not work. The help [https://docs.mp3tag.de/actions/guess] says that this action "Imports tag fields from a specified source format string [...]"
So does all of this mean that this "source format string" can be a combination of many tag fields and other [simply written] data?
Yes. You can use any source that leads to a pattern that you define in the next input box.
You can use scripting functions, text constants and field variables in more or less any combination.
It works similar to the Convert>Filename-Tag function but is even more powerful as you are not confined to the fixed filename.