I absolutely do not have a link, but I remember reading that the lipid nanoparticles are actually created by mechanical action (possibly fluid dynamics/turbulence). I thought that was pretty neat.

If they didn't use lipofectamine, what did they most likely use?

Derek Lowe discussed it a few months ago:

https://blogs.sciencemag.org/pipeline/archives/2021/01/11/rn...

https://blogs.sciencemag.org/pipeline/archives/2021/02/22/mo...