so there are different input types there based on what data was incoming in the specified property.
It can be number, then input would be any arbitrary number
It can be date, then input would be calendar
It can be string, then input would be dropdown with all provided values up to some limit (defined in configuration, by default it is 100)
If this limit is reached, then depending on configuration it would either switch to plain arbitrary input string or searchable list.
So if you have only dropdown with selectable values, without any option to insert arbitrary values it means that this property have not yet reached the limit of unique values and all those unique values should be there.
Which means there is usually no point in searching for other values, because they basically don't exist yet in current documents.
Hope this helps, if there is anything else we can do to clarify, let us know