No worries, glad it's helpful. I think the main things to research over the summer are the roles on offer at firms, which firms to apply to, and the recruitment process. If you understand these going into Spring Week apps then you improve your chances considerably.
First, you need to know about the actual roles. What actually is an investment banker? What is Sales & Trading? Etc. When you understand what these roles do and are like, you can think about which fit your skills and interests most. Ultimately investment bank roles are broadly split into three groups, the investment banking department - includes M&A, IPOs, deals advisory, etc (these are often split by firm sectors like consumer goods, energy sector, technology, etc), then there's markets roles like sales, trading, research, economics, structuring, strategists, etc (these are often split by geography and/or asset class so equities/fixed income/currencies, commodities/real estate etc). Then there's a host of corporate functions and support roles, these either help the two former groups (e.g. risk, compliance, operations, treasury) or they're corporate roles like HR, Legal (tech can be in both). Most grads want to be in markets or IBD roles as you're directly pulling revenue into the firm (and get rewarded heavily for doing so), rather than just helping other to do it. Ultimately you don't want to be applying to markets roles at some firms and IBD at others, sorta need to focus on one as the skill sets at personalities required are quite different.
Once you understand all this and know what roles could be a good fit for you, then you need to find out which firms offer these roles. For the IBD-type roles, you'll mainly be looking at big investment banks, smaller boutique investment banks, corporate finance houses, private equity firms, venture capital, infrastructure funds, maybe real estate - and failing all this the Big4's corporate finance/deals advisory teams. Whereas for markets roles, you'll be looking at different divisions in IB's, asset management firms, wealth management, hedge funds, commodity trading houses, etc. So once you know the type of roles you want, you know which segments of the financial industry to look at for their internship/grad programs and make a list of all the firms you may want to apply to.
After this, you'll need to research the recruitment processes of finance firms - they're quite rigorous as the rewards on offer are big. Normally there's an initial application where you fill in lots of details like your a-levels, degree subject and where, work experience, then a tonne of biographical information (note many firms have a minimum ABB/AAB a-level requirement). Then some make you sit a situational judgement test which are very easy, just gives you work scenarios and you answer multiple choice what you'd do (they're non-role specific and don't require any finance knowledge at all). Then you'll often have a bunch of tests, normally some combination of timed numerical, verbal reasoning and psychometric. If you pass that then you'll do a virtual interview, basically nobody's on the other end just the computer, generally it'll pop up with a question on the screen, give you 30secs to think then you have a minute to answer, you do this for 5-10 questions. Pass that and generally you'll get invited to an assessment centre, normally takes a whole day, there'll be group exercises with other applicants, more tests and a couple of interviews. Pass that and either you'll get an offer or get invited to a final in-person interview then an offer. As you can see, it's an absolute chore of a process (normally there are much less stages for Spring Weeks and the process can be much shorter for smaller firms), especially doing this for potentially 20+ separate firms while balancing it with your uni work and social life. But if you understand each stage well, then it's less painful of a process you should be successful enough to navigate yourself to at least a few assessment centres and hopefully get a couple offers.