On the topic of how those modern libraries manage to reproduce those effects, couldn’t they have used either pure bruteforce (i.e., recording and sampling every combination of notes hit together) or some advanced modeling of physical properties of pianos to recreate how the sounds coming from piano strings would interact simultaneously? Or even better, some combination of those two approaches (modeling and extensive sampling)?

Disclaimer: i by no means have any expertise on the topic and am just as curious about figuring out how it actually works as the parent comment.