I’ve played violin since I was 3 years old. So, I learned the letter names of each string and the finger numbers. And, whenever I heard a song, I always thought of a letter or number with each note. To me, it was plainly obvious what each letter and number was, and my brain even expanded the letter/number system to work with cello and the entire piano range.
Turns out, not every violinist has this, let alone absolute pitch (frequency in Western countries: 1 in 10 000). When I was 20, I discovered it was synesthesia, a harmless linking of senses/concepts in the brain.
That seemed to check out since people’s faces often have smells/tastes associated with them. Mine tastes like white sugar out of the packet. My brother’s is hard boiled egg. And whenever someone mentions a particular date on a particular year, I envision a giant number line.
Also, letters have emotions. E is happy, but lowercase e is even happier. Capital N is happy but lower case n is sad. And the months of the year go in a giant counter-clockwise circle, with my birthday in October being at the bottom. 2 is sad. 3 is happy.