I think it’s easier to explain and understand if it’s broken down.

So,

Homogeneous Mixtures

• Mixtures with components that are not visible to the naked eye and cannot be separated easily
• They have particles that are often at the atomic or molecular level size
• The whole mixture is in the same phase
• They have uniform composition all throughout the mixture

Heterogeneous Mixture

• Mixtures with components that can be easily seen even with the naked eye and separated
• They have large particle sizes
• The mixture can have substances of two phases and separated layers
• They have a composition that may vary throughout the mixture

Air is a mixture of various gasses, such as Nitrogen (78.9%), Oxygen (20.95%), Argon (0.93%), Carbon Dioxide (0.04%) There are also trace amounts of many other like neon (Ne), helium (He), methane (CH4), krypton (Kr), hydrogen (H2), nitrous oxide (NO), xenon (Xe), ozone (O3), iodine (I2), carbon monoxide (CO), and ammonia (NH3) in the atmosphere.

So, which description fits air better?

• Mixtures with components that are not visible to the naked eye and cannot be separated easily – ✔️ we can’t see the air and easily separate the many gasses in it easily
• They have particles that are often at the atomic or molecular level size ✔️ that’s why we can’t easily see and separate them (first one on the list)
• The whole mixture is in the same phase ✔️ it’s mainly gas all over
• They have uniform composition all throughout the mixture ✔️ the gases are mixed well in there and there’s no clear separation

So, the air is a homogeneous mixture – mostly.

I say mostly because sometimes it can also be considered a heterogeneous mixture when one of the gas in the air in the form of liquid (depends on temperature)