I don't understand the rights or not of crossing or not crossing a road as a pedestrian
It's nothing like as complex as it's being made out.
It's really quite simple - if a pedestrian is crossing a road (assuming it's not a motorway, or other type of road where pedestrians are not permitted), regardless of whether they are on a crossing, or not, have priority at that point. That means that drivers who sound their horn, or rant and rage are doing so from a position of ignorance - or perhaps that the pedestrian didn't ensure that the road was (or would have been) reasonably clear before crossing.
Assuming no crossings, if a pedestrian is on the pavement, vehicles have priority on the road.
What isn't catered for, easily, within those quite simple concepts, is runners crossing roads quickly, pedestrians just suddenly taking a wild hair and darting into the road (as a consequence / extrapolation of those suggesting - "Well I got their first, I have priority..." - this scenario is a suggestion that merely being in the road, suddenly, does not automatically mean approaching or colliding traffic are in the wrong), and vehicles driving too fast for the conditions, or into other roads, then behaving like somebody has taken the jam out of their donut when they suddenly discover pedestrians crossing and in their way.
I just tend to use crossings or make sure nothing's coming.
Ah, common sense, it has been a while, I've missed you...
I've only been involved in three cycle related accidents, one was when a driver opened their door on me as I was passing their car. The driver in question was exceptionally old and could barely see over the steering wheel, so presumably the side mirrors were also an issue. In fairness he was very nice, especially as I'd just totalled the door of a brand new BMW.
Well in fairness, it very much sounds like he was in the wrong, and cosmetic damage to a door could have been the least of the damage.
Near misses, or worse, collisions when somebody suddenly opens a car door, makes for heightened observation when passing cars - I always look for evidence of somebody inside, and / or give cars a wide berth where conditions allow.
The second was on my daily commute to Heathrow Airport, I was in one of those "ghost" islands turning right a I got shunted by a double decker bus. The driver was very contrite although her excuse was that she couldn't see me as she was only looking for cars.
And my experience tells me that's not an isolated thing- people only really looking for cars / vehicles. A contributory factor in me no longer being particularly comfortable cycling on the road, these days.