JabRef autofills fields that the user does not (and 99% of time does not want to) fill, e.g., isbn, url, abstract etc, etc.
The result is a messy bibtex file, ugly reference section, and a poor user experience. This has been an issue for years, an example case here: https://sourceforge.net/p/jabref/discussion/318825/thread/09ebe6ef/
Could someone briefly tell me how to only enable user-input and disable any other behavior Jabref is doing on its own?
Maybe by updating your JabRef version?
In fact, autocompletion was disabled since version 4.1, dating back to 2017-12-23. See https://github.com/JabRef/jabref/blob/v4.x/CHANGELOG.md
JabRef never ever fills in additional information if you don’t tell it to do so. If you manually create and edit entries, then only the fields that you add are there. Of course, if you use fetcher etc than the maximal amount of information provided by the online services is added to the entry.
Usually, you can simply remove/ignore these additional information that you don’t need using bibtex/biblatex facilities (especially with biblatex this is really easy). Another way would be to use the set/edit/remove feature to globally delete fields from every entry.