Much more detail on how this works together with lots of other options to configure Dropdown boxes available here: Hiredgun.tech: Power Apps Dropdown Boxes. The Dropdown box has the AllowEmptySelection property set to true so the Dropdown is empty by default
The two Text Input boxes that define the Min and Max surface are size have their Default set to 0 and 10 respectively. Other than that their properties are just as they are ‘out of the box’
The filtering of the Gallery from the selections is performed by the code below in the Items property of the Gallery
If the length of the value in the Dropdown box is zero then this indicates that a selection has not been made. When this is the case, the source for the Gallery is the full Country table so all records are displayed. I’ve chosen to show all the records because their total number is relatively small. If the table of data is much larger then a better option could be to not display any data till an option has been selected. Incidently, the IsBlank function could have been used here in place of the Len function
Once a selection has been made from the Dropdown box and therefore the length of the value selected is greater than zero. The table is immediately filtered on both the Continent and Area columns