Can I make a giveaway specifically for my subscribers/followers?

You can by making them your whitelist, though u have to control who is on that list.
Or u can make a private group on steam and make the GA Group exclusive.
Or you can make an invite only GA and send the links to only your subscribers, but they have to be responsible to not leak the link.
Or you can just share with everyone on this site :3

Invite only GAs, group GAs, or just white list GAs.

You are not allowed to create a thread advertising your Youtube/Twitch or asking people to subscribe to you. You can advertise it in the description of your giveaways, but then anyone can enter because you can not make a requirement that they subscribe for their entry to count.

You can create invite only giveaways and then somehow distribute the links to your subscribers outside of this site.

You can create a Steam group where you will do the giveaways and only allow users to join that you have verified to be a subscriber. I'm not 100% positive, but I don't think you would be allowed to create a thread in "group recruitment" for this group because there is a requirement to subscribe to you and advertising your social media in a thread is not allowed.