You've hit on my biggest gripe about vinyl siding--the vast potential for shoddy work. I'm not saying this job is shoddy, because it may have a perfectly viable drain plain behind it, constructed of window flashing tape and house wrap membrane. I can't say. But it looks a bit shoddy. That's the nature of the beast when cladding a home in thin plastic, especially if it's a retrofitment. Even work done carefully often ends up this way.
To address the situations in your photos...
This looks fine. The horizontal channel properly laps over the vertical channel and is bent over it on the flat. The vertical channel has a nice angle cut to simulate a classic miter.
This maybe could've been done a little better, with the channel more closely following the roof edge. Let's face it, though... rain can get behind almost any such channel if blown around a bit, so it's mostly an aesthetic thing.
Same here. Ideally the channel follows the window sill more accurately, but that would result in very small pieces which aren't secured well enough to stay in place. It's a bit of a Catch 22.
This is another problematic situation from an appearance perspective. The window trim acts as a channel itself, so installing blue channel inside it could look odd. Vinyl siding is almost never caulked, so there's not a great fix.
As I said earlier, this comes down to what's behind the siding. Vinyl siding is not really to be considered weatherproofing. It's mostly an appearance product that reduces the task of the actual drain plane somewhat and protects against UV damage. What's behind it keeps the real weather out.