In Manual exposure mode, TTL still meters the flash and automatically adjusts flash power to deliver what it thinks is the correct flash exposure for the metered subject area.

Your camera settings control the baseline exposure, while TTL adds enough flash to match that baseline:

• aperture and ISO directly affect how much flash power is needed
• shutter speed mainly affects ambient light, not flash exposure, as long as you stay at or below sync speed
• if you open the aperture by one stop, TTL typically reduces flash power by about one stop
• flash exposure compensation still lets you bias the TTL result brighter or darker

A common way to use this is to set Manual mode for the ambient light you want—often darker than the scene would meter without flash—then let TTL illuminate the subject. For example, you might choose shutter speed near sync speed, aperture for depth of field, and ISO for desired ambient/background brightness; TTL then supplies the needed flash.

So yes: in Manual mode, TTL does not stop working. It still automatically controls flash output; you simply take manual control of the camera’s ambient exposure settings.