The Statement ' // IQUERYABLE\<T> - almost all collections implement this, which gives you a lot of // very useful Map / Filter / Reduce style methods ' Should state: ' // IEnumerable\<T> - almost all collections implement this, which gives you a lot of // very useful Map / Filter / Reduce style methods ' see: https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerable-1?view=net-8.0#methods and the Collections that dont implement IQueryable\<T> but implement IEnumerable\<T>: List: https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1?view=net-8.0 Stack: https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.stack-1?view=net-8.0 Queue: https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.queue-1?view=net-8.0 Dictionary: https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2?view=net-8.0