WiFi Antenna Extenders: What Antenna Design Changes

Yes. Some models have internal antennas and others use external ones. The design and radio specifications matter more than the visible number of antenna elements.

Not automatically. External antennas may allow different placement or orientation, but usable performance also depends on the radio, router link, interference and building layout.

Only if the specific model has antenna connectors designed for that purpose. Many plug-in extenders have fixed internal antennas, so check the manufacturer specification before attempting any modification.

No. Antenna count by itself is not a range measurement. Compare manufacturer coverage guidance and the radio bands, and remember that walls and placement affect the signal.