You might be surprised how easily one opening can turn into something similar to a totally different opening. Also, the more you know, the more you can discover at the board in analysis.
So the answer is both. You need to know the standard plans... but if that's the only thing you know, then when something is different enough you won't be able to adapt. Or when the position changes you won't be able to change plans.
Soltis's Pawn Structure Chess is, IMO, a great book for standard plans.
Bellin and Ponzetto's Test Your Positional Play is a great book for looking at positions you may not be familiar with, and analyzing them to come to a conclusion.
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That said, when a commentator says "white needs a plan" they usually mean some combination of: 1) Black's plan is easy and obvious, and 2) it's not clear how white should proceed (and white can't wait and do nothing). They're not necessarily saying the plan should come about one way or another they're just noting that there needs to be one.
if you watch GM instructional videos, they will often stop at late opening/early middle game phase and say one side or the other needs a plan. The implication is usually that the player analysed the position and built the plan on the fly. But it's also possible that the player was simply aware of "standard plans" in the position and thus did not apply much (if any) creative thought. Obviously it's important which way it actually works for an improving player. E.g. try to absorb standard plans via game review, or learn to analyse arbitrary positions. Which way is it?