Help with when to use el/ella/ud vs tu?

Hi everyone, I tried searching for an answer before making this but I couldn't find any, so sorry if somebody has asked this before. Right now I'm using SpanishDict and their lessons to help with my Spanish and it's been great so far, but I get a little confused with sentences like these:

"Why didn't you come to my office yesterday?" "¿Por qué no vino ayer a mi consulta?"

or even online I see people tweeting stuff like

"Say your best impostor phrases" "Digan sus mejores frasez de impostor"

I thought that you would use viniste, not vino, right? They're asking that one person why they themselves didn't come and not somebody else? Same with decid instead of digan, using the vosotros pronoun instead of el/ella/ud because they are speaking to a group of people?

sorry if this question comes off as silly(probably does) I've just been confused lately and was hoping somebody could explain it to me. thank you!!

submitted by /u/Regret-Master
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