Dynamic dropdown select values depending on other custom field value

No plugins. You’ll have to build it yourself, using a JavaScript library.

Basically, you’ll need to use an onchange() event on the main dropdown, then get the value and check an array of values for that specific value.

If you search for a library, look for “chained select” or “linked select”.

Another solution would be to have all the dropdowns on the page, hidden, and then, based on your selection, show the one that corresponds to your selection.

Note: I can’t give you a proper solution as it depends on how your values are being read (manually, from array, or from the WordPress database) and it’s opinion based.