I can’t speak for YT but as far as Instagram goes, if I’m not consistently putting out good content, I’ll lose more followers than I’ll gain. I think the algorithm pushes the posts to people who are likely to engage with a post in whatever the algorithm determines is the audience. So you will get better results from a paid ad than organic growth alone. I’ve had plenty of posts get 300k+ impressions organically and only gotten maybe 30-50 followers from it. I’ve also done $20 promotion and gotten 40 followers.

I’ve noticed that putting out Reels every other day has kept my impressions on my posts have done quite well