It's a relaxed or as difficult as you want to make it.
There's nothing crazy about a day at uni, provided you aren't backed up with classes/labs from 9am to 8pm (it can happen, but rarely). So yeah, turn up to class, another class, go eat or chat with whoever, go back for another class, go home or stick around if you want. It's flexible, and you pretty much do with the day as you want in all honesty.
For me, my uni friends are more closer to me than high school friends, purely on the basis that we have more common interestes and similar aspirations so we got along really easily
Like OzKo mentioned, if you tend to see the same people in the same labs/tuts etc you'll get a similar high school style bond happening because of the frequency you see them. Plus if you have the time, and you're in any uni clubs or socities, then you'll be friendly with the people over there too and see them on a somewhat regular basis.
You socialise as much or as little as you like. If the only socialising you do in the day is getting the group work done in the lab, or talking to someone beside you in a tutorial, then so be it. Not everyone is there to make friends, as cold as that sounds, so not everyone is chatty. Not to say everyone is a snob either.
Hmm, you can have the opportunity to be close to a lecturer, professor, lab assistant etc, that's entirely up to you. Just realise they have a busy time dealing with 1000s of students a week. If having a close bond with your teachers is an important part of learning for you, then I suppose you can certainly try, but there are all sorts of professors/lecturers out there. Some of them are downright bastardly, and some just can't wait until the lecture is over so they can run out of the room. However, yeah they've got a massive workload so even if you do come across a great mentor, they just may not have the time themselves.