Changes you have made in Kodi only live in Kodi, which is why I would highly recommend using an nfo file to keep your custom metadata.
You can think of it this way, a scraper will pull info from a website (in this case IFDB) and you can edit it however you like in Kodi. But anything you edit in Kodi is only saved there. So if you ever wanted to use something else, your changes would be lost. If you make an nfo file (it's just a small file that sits in the folder with your movie) with your custom metadata, you can put that in any program and all your custom metadata will be there.
Media Companion or many other programs will let you create or edit an nfo file. For me, I create one from scratch. First I tell Media Companion that this is the original theatrical movie, so it loads all the proper actors, taglines, director, etc. It'll also scrape all the posters and backgrounds. Then I manually change the title, release date and replace the summary with info from IFDB. I also replace the poster with the fanedit one. Sometimes people summarize more in the intent section, sometimes changes, etc so I grab whatever makes sense from IFDB. This way in Kodi the movie has all the actors and art, etc. It's identical to my theatrical movies.
I used to use Plex and found that the scraper worked but I still had to tweak almost every one. Whether to add more summary or edit something. Every time it was the poster at least (the ones on IFDB are quite small and thus low res) so I stopped using it and just edited the info myself. Thankfully I didn't just edit the info in Plex, I built my own nfo file so when I started using Kodi all the information was there already.