I know kv has to do with motor rpm, but what is the better buggy motor a 1900kv or 2200kv ? Building a new e buggy

Depends on what you're doing with it. Indoor? I would stay with the 1900kv, it should be more than enough. Outdoor big track? Probably jump to the 2200kv.

I run 1800kv in my buggy. I am happy with it.

Could someone explain in depth why kv is and how to choose what kv for your scenario? I see bigger vehicles tend to have lower kv so is it a torque thing? I had the same question

Kv is essentially unloaded motor RPM per volt. For example a 2000Kv motor on say 8 volts should spin about 16,000 RPM with no load.

Note by itself Kv is only part of the power equation, torque needs to be considered as well. Power, what we are all after, is ~ RPM times torque (with proper units and conversions for exact value).

Within otherwise similar motors it works out that a higher Kv motor has more overall power than a lower Kv motor. Comparing motor selections from one line of company motor offerings provides the most accurate comparison. Note higher Kv goes along with lower number of turns for motors that are rated that way.

Motor size, number of poles, and battery pack voltage also come into play, along with a few other things like rotor size, timing advance settings, etc. This becomes more important when comparing different sizes and types of motors. The bigger 1/8 scales typically use lower Kv motors than 1/10. But they are larger can and rotor motors, frequently with more poles, which all contributes to more torque. And ran with more voltage, so more power overall.

How to choose the right one for your situation? No straight forward way other than experience really. No experience, asking others like here is a good starting point. But remember driving styles, conditions, expectations, etc, can come into play as well.