Whether the lecturer will ask questions and pick on people to answer them varies a lot; some lecturers do this as standard, others really avoid it, some don't ask questions to the group at all (and just talk at you for an hour). Generally there isn't a seating plan per se, although usually over the course of the term certain people tend to sit in certain spots regularly (e.g. the same few people tend to sit at the front, same few at the back). Usually different modules will have different lecturers, although often lecturers teach more than one module at a time (but rarely more than one from the same year, so it's unlikely you will have the same lecturer for multiple modules in the same term generally).
You will generally be expected to attend for the full lecture, although many people don't attend lectures (or don't attend some lectures - 9am lectures curiously always seem to be less filled than others...). Leaving lectures early would not usually look too good, although sometimes it happens if you have an emergency come up or something. If you are just bored or something then (aside from spending some time examining why you are doing that course in the first place) it would be better to wait for a break to be announced by the lecturer before leaving and just...not come back. It may or may not be noticed depending on the size of your cohort and how much your lecturer pays attention to individuals. You can wear whatever you want in theory, and people can and do show up to lectures in pyjamas, although I'd suggest not wearing anything you wouldn't be happy to be seen in public outside of uni wearing. If it's a guest lecture by an industry professional or similar, then you might want to not wear a onesie or something as well, on the offchance you later meet them at a networking event and that is how they remember you. .
Seminars/tutorials are usually small group classes, with ~20 or fewer students (sometimes as few as 5-10, or for Oxbridge as few as 2!), with seminars usually being more discussion based with the expectation that the students will lead the discussion (in theory anyway, in practice I'm not sure this happens that often...). Lectures tend to be larger affairs with the full cohort in the lecture (which depending on your subject and where you are studying, could be anywhere from 20-200 students), and usually a lot less interactive (but if you are on a smaller course or in certain subjects where there might be more incentive for the lecturer to ask questions to the students to facilitate learning, there can be some interaction). Often lectures can be, as above, just the lecturer reading off a slideshow to you, basically talking at you, for the length of the lecture. Better lecturers tend to try and make things more engaging, usually by including more interactivity. In principle usually students can ask questions on the material being lectured to clarify a point, although if it's very tangential then it might be recommended for that student to ask in office hours or during a tutorial or sermina.
Drinking in lectures is usually fine, although I'd recommend sticking to water (sometimes it's a rule that you can only drink water in lecture theatres at some unis), as carbonated drinks make noise when you open them and drinks other than water can leave sticky patches if you spill any. Eating is usually going to be frowned upon except in very small lecture groups where the lecturer encourages it explicitly, and as with the drinks may not be permitted in the lecture theatres as a rule (although unlikely to be generally followed), although if you have e.g. diabetes or a similar medical condition that would require you have a snack from time to time then that would probably be ok (although best to let the lecturer know beforehand).
Generally the golden rule applies, act how you would expect to be treated by your peers. People don't want to go to a lecture to listen to someone munching on crisps, or to put down their paper and pen on the desk surface and find that someone spilled some sticky soft drink on it in the previous lecture and now there is gunk all over their stationery. Note the above about food and drink is for lectures, and different rules may apply for seminars or tutorials (which may be run more informally with tea and biscuits or something potentially, although this seems unusual to me in my experience), and for obvious reasons you can't eat or drink in laboratory or computing environments normally (labs you can't ever eat or drink in, computing rooms usually you are limited to only drinking water from a covered container, if that).