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.