Learn English – Verb for understanding something so well that it becomes part of your nature

Certainly acquaintance would be my own choice, but perhaps you found something remiss with it.

You might further qualify it by saying something like casual acquaintance, incidental acquaintance, nodding acquaintance, or long-time acquaintance.

Of those, I rather like nodding acquaintance, since it implies that this is someone whom you would nod to in passing, but not pause and chatter at.

function

noun

the kind of action or activity proper to a person, thing, or institution; the purpose for which something is designed or exists; role.

Dictionary.com

This edit is from AndyT's comment. It gives you another noun to use that more closely matches the requested usage of qualifying whether or not something is working as designed. And from the discussion in the comments, I now prefer functionality over function.

functionality

noun

1 The quality of being suited to serve a purpose well; practicality. ‘I like the feel and functionality of this bakeware’

1.1 The purpose that something is designed or expected to fulfil.

Oxford

This edit also stems from the comments and discussion. It provides different parts of speech to use in sentences that are structured for such.

function

verb (used without object)

8. to perform a specified action or activity; work; operate: The computer isn't functioning now. He rarely functions before noon.
9. to have or exercise a function; serve: In earlier English the present tense often functioned as a future. This orange crate can function as a chair.

Dictionary.com

functional

adjective

1. of or relating to a function or functions : functional difficulties in the administration.
2. capable of operating or functioning : When will the ventilating system be functional again?
3. having or serving a utilitarian purpose; capable of serving the purpose for which it was designed

Dictionary.com

Common usages of these words in everyday speech:

"This phone is ugly, but I love its functionality."

"How well does this computer function?" "It is old, so not well."

"Your brand new Ford breaks down all the time. I drive an '85 Dodge, but at least it's functional."