Earlier quoted context omitted.
Not so for the default mode in LaTeX. It tries to guess which periods indicate end-of-sentence (based on whether the next letter is capitalized) and will add additional horizontal space accordingly, unless you tell it not to by declaring your document uses “French Spacing”. (Edit: and my memory may be serving me incorrectly here, but I recall 20 years ago having a support package that also detected double-space in th…
I remember reading the TeXbook and learning that the space after a period is wider if there's a capital afterwards and the various ways of suppressing this behavior. So it's a TeX thing, not LaTeX.
https://stackoverflow.com/questions/2024338/latex-sometimes-...
LaTeX is based on TeX