It's not a bad advance to make in one day. They'll likely advance further soon.
Trenches and forts are usually only around strongholds (large urban areas, like Damascus was), otherwise it's usually barricades and minefields. The whole war is more about hit and run tactics and more elastic form of defense, at least in the countryside where it's just not viable to bunker down.
>do they just take some land, sit there and push again the next day
Mostly, yeah. Fighting at night is fairly uncommon. Of course, while they sit in place, the airforce and artillery keep bombing the enemy.
>Is the entire front covered or are there defensive gaps?
Far as I can tell, jihadis don't have the manpower to really guard the hwole front. So they will only leave nominal presence in border villages (since they don't know where SAA will attack next), and then the offensive starts, they'll start moving troops from everywhere else to counter it. As you can tell, it's not very effective, but it's about all they can do – it's not 2015 anymore, and jihadis are outnumbered, with worse gear, and only supported by Turkey, which appears to already count Idlib as lost and instead focuses on Libya.
>How are logistics handled?
I don't think they're really a factor here. Jihadis control a fairly small area now to the point everything's close enough not to need supply trains or similar. They got various stashes or even proper depots to pick arms from when needed and that's about it. As for where they get war material, it's Turkey, of course.