WASM is just like assembly. It has instructions similar to MOV, JMP, STA, etc. It can be distributed as the textual instructions or as the compiled binary format.

When it started it was interpreted by JS or could be compiled to JS directly. It proved to be faster than hand-written JS. However, it still had to go through a JS interpreter. Now, there's a WASM interpreter / virtual machine built into browsers. It's very much the new java bytecode but without running an unsandboxed, external (outside of the browser) java virtual machine.
Given it's an intepreter / virtual machine, it of course has limited APIs in the browser. For a while, it was not possible to access the DOM from WASM, so JS would do the DOM stuff and WASM was called (just like calling an external function in a lib in C/C++/Rust/...) upon to do computationally complex stuff since it was faster than running it in JS through the JS interpreter. IINM, WASM now does have access to the DOM.

Of course there are WASM interpreters outside of browsers that can be included as libraries in other languages. Rust devs are using it for example for plugin systems in their software.

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