I have an old 20k IG account that I built up a few years ago, in 2016-2017, using the subscribe/unfollow method + buying reviews on IG. I think 40-50% of followers use the Follow/Unfollow method. But that's a bygone era. Nowadays you can just buy instagram likes cheap and followers will automatically pull up on your account. Also, I've heard that the IG algorithm doesn't like these types of accounts (follow/unfollow accounts) and they earn a bad reputation and limit reach and are inactive for a long time, so that can add extra reputation to the algorithm.