You might want to ask some event riders to a lunch with you (sitting 6’ apart of course, or maybe a Zoom lunch), and you’ll be able to hear, ask, discuss and get some great anecdotes as well. And get an idea of what to expect at your closest eventing venues. 
For divisions/classes you enter, there is the general level of Beg. Novice, Novice, and so on. And schooling horse trials will offer even lower divisions, and sometimes a “speed bump” division that is just logs and a few terrain features.
In addition to those levels, you also enter according to your classification as a competitor. Horse (green horse division, rider can be any classification); Rider (a rider who is not competing above the level; horse can be any classification); and Open (any rider, any horse). So you can be in the BN Rider division with other riders who aren’t competing above BN (in the last 2 years).
On XC, everything at BN and lower unrecognized levels is made to be basic and straightforward. There are no combinations, officially, although there may be separate jumps that are placed with a related distance between them. That is, close enough that you need to have the next jump in your sights to get there accurately. (Even lower unrecognized levels may have jumps spaced so that each is completely independent of the others.)
Beg. Novice will have a “ditch” that is usually a “barely there” ditch. Roughly, a couple of timbers on either side of a shallow spot. A maxed-out BN course might have a deeper ditch, with a separate jump a few strides before the ditch.
The Beg. Novice water is just trotting through the pond. You can also walk or canter through the water, as you wish. At BN there is likely to be a jump on dry ground before and/or after the water.
You also go down a low bank. There is a technique for riding this, but at BN often you can just use a good seat with your feet in front of you, and don’t lean forward, and the horse will go fine.
One of the best things about XC is that there is space and lots of opportunities on course to make some adjustments, even throw in an extra circle away from the jumps, if you need to do that to keep it all together. A bobble here or there isn’t the end of the world, there is plenty of opportunity to fix it.
Talk to local eventers to learn about your local horse trials and the level of course each offers. Basic and inviting? Or a maxed out championship course? That can change at each venue from year to year. As an example, a maxed-out BN course might have a jump before and after the water (all numbered separately, not a combination). Whereas a basic course may have no jumps close to the water, but will usually have one several strides from the entrance or from the exit. In another example, a basic course will have only a few jumps that are fully up to height, whereas a max-course may have all jumps at the max height and some at max width as well.
An eventing instructor who is very encouraging at the lower levels can help you understand how a course is laid out, and what to expect of the level of the jumps.
I strongly encourage an instructor/coach who is a true eventing instructor with a long track record in eventing. There is so much to know about riding to jumps going up and down hill, on various terrains and footings, how to ride the jumps on a longer stride, how a horse will read related jumps and terrain, and so on, it really takes a full immersion to be ready to teach others to event.
Here’s a link to eventing rules, for comprehensive answers. I am pretty good at reading technical instructions, but I found this document to be dense until after I had been to some horse trials and started to get a feel for how they run. Then it became fascinating!
https://www.usef.org/forms-pubs/KlV5P9prkmM/ev-eventing-division
