The default UITextInputStringTokenizer prefers to place the caret at word boundaries. I implemented exact placement for my coding app, Codea, and wrote about the text system extensively here: https://sim.coffee/textual-healing/

Exact caret placement is actually surprisingly effective with a reasonable font size. I wish Apple offered it as a preference system-wide

Selection defaults to a double-tap to select at the word granularity. Triple tap will select at the paragraph granularity. This is customisable in your own apps, though the system defaults to the one specific to your current language