Which standard C++ features can be used for querying machine/OS architecture?

Question

What are the standard C++ features and utilities for querying the properties of the hardware or operating system capabilities, on which the program is running?
For instance,

Answer 1

how do you detect how much RAM the computer has, or how much RAM the process is using, or how much disk space is available to write to in a certain directory, or how much L2 cache is available?

You don't. Precisely none of this is the purview of the C++ language, which describes an abstract machine.

The only reason it tells you the number of cores available is because otherwise its multi-threading model would be close to useless, and even then it does so in an abstract way ("hardware concurrency" is not "number of physical CPUs in your desktop PC").