1/ The followers usually come from sponsors. For example, Kylie Jenner promoting the giveaway on her IG will say to her followers to go follow all people that the @giveawaypage follows . I personally have also developped a technique based on giveaways that doesn't need any sponsor and brings a great amount of followers.
2/ Lots of comments? Usually it is part of the rules to enter the giveaway. If we take the example with Kylie above, she will say to her followers someting like "to enter the giveaway, go follow all accounts @giveawaypage follows + mention 3 friends in the comment section"
More than half of subscribers from other bloggers after the contest unsubscribe. You need to cling to the content, then there is a chance that most of the subscribers will remain and will be active, but the activity depends on the correct content.