Convert list of single nucleotide coordinates (hg38) to nucleotides

See the FAQ: How to do a minimal reproducible example reprex for beginners. Unless someone with biochemical domain knowledge comes across this, it may be unlikely to get answers.

With a reprex, however, it may be possible to attract the attention of general users.

Every R problem can be thought of with advantage as the interaction of three objects— an existing object, x , a desired object,y , and a function, f, that will return a value of y given x as an argument. In other words, school algebra— f(x) = y. Any of the objects can be composites.

Here, x is a list of typeof chr, and y is a list of typeof chr. What is needed is f that that relates one to the other. For that, some domain knowledge is needed.