Static fields may be placed on Frozen Object Heap. The values of static readonly fields may not exist at all if the ILC's static constructor interpreter can pre-initialize it at compile-time and bake the value into binary or codegen. Tiered Compilation does a similar optimization but for all cases. This is with JIT though which is not usable in such environment.
Otherwise, statics are placed in a static values array "rooted" by a respective assembly. I believe each value will be contained by a respective box if it's not an object. This will be usually located in Gen2 GC heap. My memory is a bit hazy on this specific part.
There is no concept of globals in .NET the way you describe it - you simply access static properties and fields.
In practice, you will not be running .NET on microcontrollers with existing mainline runtime flavours - very different tradeoffs, much like no-std in Rust. As mentioned, there is NanoFramework. Another one is Meadow: https://www.wildernesslabs.co which my friend is using for an automated lab for his PhD thesis.
Last mention goes to https://github.com/bflattened/bflat which supports a few interesting targets like UEFI. From the same author there's an example of completely runtime-less C# as well: https://github.com/MichalStrehovsky/zerosharp. It remains a usable language because C# has a large subset of C and features for manual memory management so writing code that completely bypasses allocations is very doable, unlike with other GC-based alternatives.
