Part of learning an instrument is physical: teaching your fingers where to go when, and refining your timing and precision as you manipulate the instrument.
However, the other big aspect to learning an instrument is mental, and most of that mental aspect is common across instruments. This is the “listening skills” and “musical understanding” which lie beneath instrumental skill, and once you have developed it for one instrument, you will find it transfers easily to other instruments.
That may seem vague and abstract, so let’s consider some practical examples.
Knowing which notes to play
Developing your sense of relative pitch through ear training gives you an instinctive understanding of which notes “belong” in a given musical context and which ones to play when.
As soon as you begin improvising or adapting the music you play, you rely on a combination of theory knowledge (e.g. the key signature) and listening skills (e.g. interval recognition or solfège) to know which notes you want to play.
Actually playing these notes comes down to your instrument-specific skills: for example, knowing which key on a piano is F#, or how to play a D7 chord on guitar. But knowing which notes you want to play is all in the brain and the ear.
Your sense of rhythm
Accuracy of rhythm and timing has a major impact on how polished your performance sounds, and it is only partly about training your fingers. You must have the motor skills required to play notes with accurate timing, but even before that you must understand in your brain how the rhythm fits together, and have the ability to maintain a metronome-like sense of the beat in your head