mmap can be used for a wide variety of reasons; as a specialized memory allocator to creating shared memory for IPC, to faster and/or more convenient fs file reads/writes, so it is a big space and hard to guess where you should start.
Check out the memmap crate. Last I knew, some improvements to the API rustdoc had never been released, so you might possibly want to look at the linked (github repository) source code.
On this forum there was also: How unsafe is mmap?