In my various rambling thoughts about ability I think it is useful to have some more precise definitions. Of course this is somewhat changing them from their accepted definitions which are pretty interchangeable

Skill - capacity for an isolated task that, other things equal, is virtually linearly correlative with time investment/reptition. A robust definition might be "something that can be practiced to the point of unconscious competence". examples: basic literacy, touch typing, kickflips, riding a bicycle, walking, backflips

I think knowledge might play a bit of an interesting role - researchers love to study chess players and an idea that comes up quite a lot is that they have developed an extensive library of games that forms the basis for their ability to read the board in chunks like "lines of force," and so on. I imagine the games themselves are partially synonymous with heavily insulated myelin--recalling a game calls on these chunked patterns where each move influences what comes next, perhaps in the same way that we recall the chronological ordering of events in a book. Then the two are quite woven together. So perhaps knowledge is nearly quite directly skill. At the least all experience and information pertaining to a discipline would be categorized under this umbrella.

This is somewhat distinct from crystallized intelligence (Gc) since that correlates with fluid (Gf) and general intelligence, but similar in the sense that most of the trait derives from the base of "experiences".

Talent - the same as the commonly-agreed definition: whatever innate traits you possess that offer an advantage of some kind in the activity, whether that is increasing your potential ceiling of performance or capcities that will help you acquire skills faster. This encompasses a wide range of things, from the expected such as intelligence in the statistical trait sense (g / Gf), to the slightly odd-er in the form of positively contributing life experiences (good mental demonstrated by a parental figure, etc). Raw intelligence is far from the full picture of talent itself, let alone ability as a whole. Your experiences affect how you respond to failure, approach various types of problems (for one reason or another you may have seen one in a similar form before), where you derive your sense of drive, and inform your attitude and understanding of the discipline, to name a few. more examples: large wingspan for swimmers, being 5'2 to 5'6 for a male gymnast, sub-250ms reaction time (a low bar, but relevant all the same) for pro gamers, a unique manner of observing the world for art or writing

Fitness - accumulated physical capacity for an activity, distinct from skill or talent. Maybe it can be said this is "skill for the body". Well, skill is entirely physical too as it basically boils down to myelination but I think the distinction still makes sense... examples: acclimiation to repetitive stress on the knees, ankles, shins for long distance runners, ability to hold a consistent rhythm for extended periods of time in rhythm games and similar, aerobic fitness which applies to many sports

Ability - the overall demonstrated proficiency in a discipline. More or less the idea of "skill" in its typical usage. This is the combination of skill and talent. examples: the ability to paint realistically, or read sheet music, etc