Hans Zimmer did an interview a few years ago (I can't remember where, but maybe you can find it) where he said that when he was working on the score for Inception, he kept imagining the guitar part being played by Johnny. I guess he was a fan and money wasn't really an issue for that project, so he got hold of Johnny's phone number and invited him out to LA. That was strictly as a session musician, playing very precisely what Zimmer had already written. Maybe it was initially just a fanboy thing for Zimmer, or maybe he invited Johnny in with an eye to future projects.
They then worked together on The Amazing Spiderman 2, parts of which feature a "supergroup" put together by Zimmer, including Johnny, Pharrell Williams, Junkie XL and Mike Einziger from Incubus. Johnny had a bit more input into this project, and gets a co-writing credit on a couple of tracks.
Johnny and Hans then worked on the soundtrack to Freeheld, which was a minor release, although it had some big names in it (Julianne Moore, Ellen Page, Steve Carrell). There doesn't seem to have been a lot of budget, so it was recorded very quickly, with Johnny improvising guitar to the rushes and Hans then coming up with arrangements around those improvisations.
The original composer for No Time To Die dropped out late on, and Zimmer (apparently friends with Barbara Broccoli) agreed to step in on condition he was allowed to hire a collaborator. My guess is that he imagined working in a similar way to Freeheld, as a way to get the score done quickly, because he only had a window of about a month. That shows pretty high confidence in Johnny, for such a high profile project.
As far as I am aware, though, Johnny and Hans have not been directly involved in Billie Eilish's song, although I suppose it is likely that they will have had input into the choice of artist and OK-ing the song. Maybe they will have also met with her to discuss what they were doing with the score, so that she would have an understanding of what she had to fit into. But it's traditional for the theme song to be a separate deal from the score.
Also, they are definitely both cucks, which explains the whole thing in fewer words.