Some quotes from this topic:
“a different shkiya”
“there are two shkios, one the normal sun set and 72 min after that is another shkiah”
” hold the shikia is 60 minutes after sunset”
This is an error. Shkia means sunset. [שקיעת החמה Literally translates to; the submerging of the sun (below the horizon).] By definition, it has little to do with how dark it is in the sky, or the appearance of stars. The question of when is nightfall, is also not directly related to sunset. Everybody who lived before the advent of electricity, had no doubt about what sunset was, and when it is. I do not know when the terminology became confusing, but they certainly did not mean to say that the sun sets twice.
For Yidden it is important to know exactly when is nightfall, for basically one topic. it is of little consequence to zman tefillah. We [with one exception] are not waiting for nightfall to daven mincha. As both mincha and maariv can be accomplished before sunset.