Yes, in general on Windows processes with higher privilege levels can get access to read/write another processes memory, or even inject code into them. And even Admin-level processes can still be broken into by something running as a service with even more elevated privileges like NT AUTHORITY\SYSTEM.
This has long been a leaky part of Windows security. If your malware can get its code running inside a highly privileged service or process, it can do more or less whatever it wants to the rest of the system. But even when not used for nefarious purposes, it is still an extremely dangerous capability in that it can be very easy to create problems .
By default, any application's memory can be read and written to by other processes running as the same user, as far as I know. The way to deal with this is to set process security descriptors, but admin can still bypass this. There are protected processes, and protected processes light, but those are not used by most software (mainly anti-malware afaik.)
Although that was definitely the intent, I actually don't know about specific things that use it. I'd love to hear what actually uses it. (I don't think Widevine l3 does, for example.)
This is wrong, on Windows there are system calls to access memory of other process and on Linux you can do it using debugging. Also on Windows there is a tradition to inject libraries into other processes, create threads in processes etc.
On Linux, ptrace permissions can be restricted [0] and some distributions do this by default.
Whether this provides any meaningful security is questionable unless you pair it with filesystem isolation to prevent malicious programs from modifying config files / bashrc / etc. Meanwhile it does make legit uses of ptrace more annoying.
