Can A Government Official Select Someone To Replace Them Temporarily (If They Are Sick Or Can't Fulfill Their Duties Temporarily)?

[CuriousAboutPoliticalSci]

Hello,

I had a question regarding US government officials.

When a government official is sick, or can't serve due to something like a death in the family/funeral, can that same government official select someone else to replace them temporarily to serve in their place?

For instance, let's say that a senator can't make a specific senatorial vote (because they recently got sick), could that same senator select someone (e.g., like an assistant who works for the senator), to vote in their place (this assumes that the person voting, votes exactly in the same way that the senator would have voted)?

If not, can I assume the only person who can vote (or serve in any elected capacity), must be voted by the citizenry (or selected by the Governor)?

Thank you,

AB