Is the onboard flash TTL? Or the only way to control the power output with flash compensation? And lastly, does TTL only refer to speed lights?
I'd say yes to all.
The camera internal flash has a menu to be manual flash mode, and another mode to be TTL flash mode. Hot shoe speedlights typically have a similar menu - but a few are TTL only or Manual only.
TTL flash means the camera meters and sets the flash exposure automatically. The way we can tweak that result is flash compensation, which changes the exposure goal to be a bit different, a bit more or less flash intensity than what the camera metered and planned.
TTL is metered by the camera, which then sets the flash power level to comply. This does require a system flash, capable of understanding the camera instructions, typically from the hot shoe. That does rule out studio flash, which are manual flash only.
There are third party speedlights, but for TTL, we have to choose the model that is compatible with the Nikon system.
Generally, speedlights used off camera are limited to manual flash mode (because no hot shoe communication to get instructions from the camera).
The Nikon commander is an exception, capable of TTL communication with certain remote flashes in its special way.
And there are one or two other third party systems mimicking TTL, so there are possibilities, but generally, off camera flashes (including studio flashes) are manual flash only. No communication capability.