A poll is any count of preferences. See Merriam-webster sense 4a
the casting or recording of the votes of a body of persons
and sense 5a:
a questioning or canvassing of persons selected at random or by quota to obtain information or opinions to be analyzed
Both ultimately derive from sense 1 "Head". A poll is in this original sense, a head-count.
The word "poll" can be used for a formal election. (I believe that it is more common in that sense in the UK.) but can be, and now mor often is, used for an informal opinion servery. That kind of poll is not usually called an election. So one might say that all elections are polls, but not all polls are elections.
UPDATE based on additions to the question:
A selection by votes among users of an app for questions such as "Which one of these movies is better?" is not likely to be cvalled an election, and if that term is used it may cause confusion, or be thought odd. Such a choice is most likely to be called a "poll", but it might be called a "vote". Calling it an "election" would, in my view, be unwise.
Elections, in addition to usually being for official or at least well-defined positions, generally have a limiterd and defiend electorate (set of possible voters). A choide in which any random person can participate, and no accoutn is kept of who does participate, is not likelyn to be called an "election".
The answer by fertilizerspike mentions the choice of a "prom king" and "prom queen" at many US high schools. This choice is indeed called an election, or least it sometimes will be so called. It is true that these are not official positions. But they are fairly well defined and recognized positions, at least within a given school. They generally have actual function, althoguh not very important ones: the "king" and "queen" appear at a particular time and place, in that role, and no one else does so. Once the vote is held and the results tallied, there is normally no debate about who in fact holds the position. In all these ways this choice is more like an election, and less like an opinion poll, and so it might well be called an election, whereas an expression of opinion on "who is most sexy" or "who is the best actor" has none of these characteristics, and would nor normally be called an election.