TikTok is like that but it’s loading videos continuously so it makes more sense that it would use a lot. I never noticed significant battery drain with Apollo.
There was a post on the mildlyinfuriating sub where someone had tiktok installed but hadn't loaded it in like a month or something, but it had used a full 2GB of data. Tiktok does more than load videos.
Almost certainly not. I think if an app gets the right permissions on installation, it doesn't have to be officially running to take data and send it to the company.