To be clear, I imagine that it’s not that the reservoir is literally “intentionally” storing different strains but just that viruses are more accurately thought

To be clear, I imagine that it’s not that the reservoir is literally “intentionally” storing different strains but just that viruses are more accurately thought of as “quasispecies swarms”: https://en.wikipedia.org/wiki/Viral_quasispecies https://en.wikipedia.org/wiki/Viral_quasispecies This is a naturally emergent phenomena. (Just adding some further context to what you were saying)