Difference between Select and SelectMany in LINQ in C#

Everyone knows what the Select operator does in LINQ but many are not aware of how useful the SelectMany operation can be.
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[@kskunk]

💯Best SelectMany explanation!! Thanks for all the many examples and code!!

[@fkbey5756]

Thank you for your clear English and your time.

[@mins_91]

Thank you very much for this clear explanation

[@kidsvlog1309]

awesome explanation ❤

[@user-wj6sw6lz3i]

Great

[@darkwhite123]

what if there are two lists? a list with won years and a list with participated years. selectMany is executed consecutive? so there will be "stupid" duplicates? do you know of a way to achieve such without duplicates?