I would always recommend researching a potential employer and being able to express some knowledge (and ideally interest too) of what they do. As for why bother to ask admin job candidates why they're interested in your university, well, because universities have a diverse breadth and scale of professional services jobs and someone starting in a low level admin job might end up staying long-term and developing their career into higher positions, maybe even senior management. A candidate who has bothered to look at your website to find out what your organisation does will have an edge over someone who just says they need a job and and it's an easy commute.
I'd focus your research on two areas: the university itself and the type of work Admissions staff do.
Look at the university website. What type of university is it (former polytechnic or 'red brick' ie an old university)? How big is it, in student numbers? 4,000 or 20,000? Understand the difference between Undergraduate (UG) and Post-graduate (PG) courses. (UG are first degrees, which will be mostly school-leavers. PG qualifications are more advanced than UG, so their students are older and there are far fewer of them.) What degrees are offered? Some might have specialist entry requirements rather than just A level results, eg Art applicants will need to show their portfolios. (You don't need to know precise details, but just be aware that some courses might have complicated admissions processes). Do they have a lot of international students? See where your university is in the UK league tables.
As for Admissions, get an idea of the application process and the timing throughout the year. (Some people think universities are like schools and are very quiet during the summer holidays. That's not the case, especially for Admissions!) Undergraduate students will apply via UCAS which is a national, centralised application portal used by all UK universities. The deadline for applications is usually in the spring. Applications will be reviewed and applicants either given an Unconditional Offer (they are guaranteed a place on the course, regardless of A-level results); a Conditional Offer (they need to achieve certain grades) or be rejected.
(Yes, I know Scotland has Highers, not A-levels.)
A-level results are released in August, and then all hell breaks loose. Universities receive them a few days before the students do, to give them time to work out whether their applicants have achieved the required grades or not. Students who did get their grades then decide whether to accept their place. Students who didn't get their grades might contact the university to see if they'll still be accepted or might go into Clearing instead, which is basically frantically phoning around trying to find a university willing to offer you a place on a course. Students who got better grades than expected might try to trade-up to another university. The universities higher up the league tables make less use of Clearing, if at all, because they easily fill their courses and don't have empty places. At the other end of the scale, some universities are desperate to attract students during Clearing. That takes up the rest of August and possibly into September (the desperate universities with lots of empty places will stay open for longer). Even then they won't know for certain which students will turn up and enol. Enrolment is important because it's at that point students commit to paying their fees. They usually enrol at the end of September/beginning of October, so it's not until later in October that the university can see how many students enrolled and paid their fees. (And that applies to second and third year students re-enrolling/paying their next fees. There's no certainty that a student starting their first year will continue in the following years. Remember, student fees = university income.) Some may drop out soon after - they get a partial refund if they withdraw within a certain time period.
Sorry, this has turned into an essay.
As for the interview itself, universities tend (although not always) to have evidence-based interviews using the criteria in the job description. So usually lots of questions "Can you give me an example of when you..." or "In the following situation, what would you do?". It's OK to take a few seconds to think of an answer. Prepare by going though the person spec and your supporting statement in your application and thinking of suitable examples. If you have time, practice explaining them out loud so that it sounds more fluent in the interview.
As PPs have said, communication skills, GDPR/confidentiality, diversity and equality (eg dealing with international students; widening participation initiatives among UK students), and team working will be important. Also being able to work under intense pressure in periods of high activity.
Good luck!