How can I filter several drop down list for overview?

Question

How can I filter a summary list of the selected names in use from the row with the drop down ? I would like to narrow down an overview of the choices.
Thank you,

Eric Maccabi · Accepted Answer

i believe above is a great solution. Then, if you want that to be a dropdown, you can have the formula above (in IceSwan's response) pasted into a separate sheet and reference that for you data validation where ever you want the dropdown to appear.

IceSwan Petchy · Accepted Answer

I'm not certain if I understand your request correctly. 
But if what you want is to "combined" all the selected choices to a single column, try put this formula in ~ cell Z9 or something
= TRANSPOSE(   FLATTEN(   A4:X9   )   )
if you want to see repeated items only once, add   UNIQUE(   ....   )

​If that does not solve your issue please share the sheet with us and manually demonstrate the expected solution in that sheet