What specifically is happening when someone is nonverbal but able to write or type?

I’m trying to phrase my question in the most respectful language, please forgive me and feel free to point out ways I can improve. I’m struggling to understand the nonverbal perspective – I’m not nonverbal myself.

I’ve met a few people with autism who are nonverbal, and the real impression I got was that they were nonverbal in the sense that they did not use expressive linguistic language at all. They were able to point to things they wanted or raise a fist if one of their personal boundaries was breached, but they were not able to express abstract or symbolic language in any form (e.g. writing, typing, sign language such as ASL or BSL, Morse code, semaphore, etc.).

I’ve read some accounts online of people claiming to be nonverbal but that they communicated online using text, which is a form of symbolic language. This is the perspective I’m having trouble understanding and empathizing with.

I’m directing this question to people who are nonverbal but who communicate in other forms of abstract language such as typing, or at the very minimum people who know someone who fits that description very well.

When someone is nonverbal but able to type, write, or use sign language, what is it that is happening when a person tries but fails to speak verbally?

– Is this an issue with fine motor control of the muscles of speech (e.g. lips, tongue, vocal cords), and any attempt to speak is unintelligible to others? As an analogy, I would imagine this to be similar to someone with uncontrollable hand twitching trying to sit down in front of a piano and play – they literally can’t because they can’t control their fingers well enough to consistently hit the key they want.

– Is this an issue not with muscle control per se, but with the skills of understanding how to pronounce various phonemes? As an analogy, I would imagine this to be similar to someone who understands what a piano is and how to push and release keys, but doesn’t know what notes correspond to what keys.

– Is this an issue not with muscle control or phoneme enunciation, but with the skill of combining phonemes to make speech? As an analogy, I would imagine this to be similar to a person who can play any note they want to on a piano, but can’t play a musical composition.

– Is this an issue with crippling social anxiety related to speaking aloud? As an analogy, I would imagine this to be similar to someone who can play, or could learn to play, the piano, but is too afraid to try.

– Is being nonverbal something else entirely?

In other words, I’m asking where in the body’s systems this happens. For example, is it anatomical? Muscular? Neuromuscular? Neurological? Psychological? Educational?

By asking this question, I’m not trying to imply that people who are nonverbal are less human or inherently broken.

Thank you for your perspective.