unlinked all references
Instead of syncing, i read somewhere that i could copy the zotero library files, put them on a usb stick and take that to a different computer. There i deleted the earlier version of the library and pasted the one i had on the usb. This, i suspect, i effectively unlinked all of my references. I noticed when i went into the library and tidied many URL's, but when i refreshed it didnt change the bibliography. However it does change when i reinsert the citation using the library. So, i need to do this throughout the doc, correct?
If you created two libraries separately on two separate computers (including via export/import), those items are totally unrelated, and citations from one library wouldn't be connected to citations from the other library. Transferring the library by copying the data directory makes an exact copy, so those transferred items would be unrelated to the document for the same reason. You'd have to reinsert each one to use current data from the library.
https://rintze.zelle.me/ref-extractor/
And just to clear up possible misunderstanding here, transferring the data directory as described in that link copies your library exactly as it is, and any links to citations in documents created against the library on one computer would stay connected on the other computer. The point is that, however you created two separate libraries originally, which I'm guessing was with export/import, created two unrelated libraries. So if you now transfer your data directory from Computer A to Computer B, a document created against B wouldn't be connected. If you had transferred the data directory in the first place, a document created against B would still work if you transferred the data directory again, because it would be the same exact library as far as Zotero was concerned (not counting any further changes you had made on Computer A).