You have to have air as heavy as water.
If these things have an average density similar to the sea faring battle ships they look like, they could only float if the air were about as dense as water.
These things fly like blimps, using buoyancy, not anti-gravity or jet propulsion, or helicopter blades or any other kind of active lifting system
You're less interested in how these airships would work than what a planet with airships would have to be like
Amazingly, for something held aloft by buoyancy, gravity doesn't matter! It doesn't matter if the gravity is as strong as on Jupiter, or as weak as on the Moon. Buoyancy only cares about how much air you displace. If the air you displace is heavier than you, then you shoot upward, if the air you displace is lighter than you are, then you fall downward. Since the pictures show ship holding still in the air, they must have an average density exactly equal to the air they are in.
So, Earth like gravity, exotic atmosphere. How do we make that happen?
The gas with the highest density (that we know of) is tungsten hexafluoride. It's colorless, which is good for your epic vistas, but toxic, which is bad for whatever lives there. It's also only 13 kg/m3. That's a lot better than the 1.2 kg/m2 of air on earth, but no where near the 998 kg/m3 of water.
So, modern material science lets you down, but don't let that stop you! Let's assume you can find an amazing, inert (or at least non-toxic) gas that's a as heavy as tungsten hexafluoride and fill your atmosphere with it. You still have to have very light ships. I'll leave it to you to figure out what a 13 kg/m3 ship is like. Mostly balloons on the inside, I guess.
A few more details
Atmospheric pressure is a function of depth and density. That means that with a heavier gas, your pressure is going to be a lot more, or your atmosphere isn't going to extend so high above sea level.
With high density air, even slow winds carry a lot of energy. Light winds could blow decently sized rocks around. Wind storms common on earth would be terrifying, city destroying disasters. Sky scrapers, which on Earth are limited mostly by wind, might be impossible on such a planet.
Floating objects feel almost no wind. Without propulsion, they move with the wind so perfectly that passengers will not perceive any wind at all. Not breezes or turbulence or anything. Hot air balloonists comment on the eerie stillness all the time. An airship under power is a different story. The wind of their motion could of course be quite substantial.
Oxygen breathing life on earth can survive in a large variety of atmospheres as long as there are no toxic gasses, and there is enough oxygen. It's not a matter of percent, it's a matter of how many oxygen molecules in each breath. You don't need 20% oxygen, you need a partial pressure of about 160 mmHg. In the past, NASA ran space ships with 100% oxygen at 1/3 atmospheric pressure. Or 50/50 oxygen/helium with about 2/3 pressure. You could have 4 atmospheres of pressure, and only have 5% oxygen and Earth life could maybe live there. Or adapt to live there, at least.
The nice thing about cranking up the pressure, is you get extra density. In 4x pressure, you get 4x density. That means your ships can be 52 kg/m3 instead of 13. That's like 10x lighter than real warships, but feels much more doable.