Core difference is that you subscribe to each channel individually, kinda like Patreon, instead of the Nebula model where you pay one fee and have access to every creator on there.
Floatplane is the video hosting service that they created and own. Instead of being ad supported it requires a monthly fee for each content creator on it (in the vein of only fans or patreon). So a loss of subs is a direct loss of monthly income for LTT.
Youtube is being an ass about guns specifically. They had vague rules about assembly instructions including not being allowed to show someone screwing in a suppressor. Then they did the typical google thing and offerred no discussion or recourse options for talking about the rules to any of the youtubers. Essentially, fuck you, you do what you're told type of deal. It was enough that all the gun youtubers had legitimate worries about their job security in the short and longterm future. They've since walked back that specific rule, but there are still a lot of other rules restricting what kind of gun related content is actually allowed. And all the channel owners know that google can and will cut their gun content on a whim without forewarning or talking about it.
You also see this with their rules about swearing in videos. Whatever makes the advertisors happiest apparently.
I mention the silencer thing because attaching one to a gun is literally screwing it in - every hollywood movie that includes a silencer has this specific scene in it, and so did all of the gun youtubers. Google never told anyone if these rules were for going forward or if their old content would get called out for it too, and so there was lrgit worries about finding a channel ban for videos uploaded ten years ago.
DankPods also has quite a few channels on there and does his drum streams there too. I wonder if people subscribed to non-LMG channels are unsubbing, figured Floatplane's attachment to LMG might cause collateral damage to creators only related through the service they're using.
I have no idea. I learned this from the not political gun youtubers and various other hobby interest channels telling me they had a floatplane account set up.