It was ~12 years ago, so there's not much left around, but here is an engineering blog post from Linkedin talking about how they architected it https://engineering.linkedin.com/mobile/linkedin-intro-doing...

I don't recall all of the specific details, but I just remember reading about it at the time and how they bypassed some of iOS security protections to do it. Adn that they didn't get perma-banned from the various app stores back then is beyond me. It's a huge part of why I avoid installing apps on my phone in general.