It varies by app. Some apps reserve a "frame" area that includes controls. Some apps take the entire screen resolution area to provide their display (usually subject to proportional aspect constraints - that is, they fill to the maximum width or height of the screen display on whichever dimension allows the "max-ing" without stretching on the relatively smaller dimension).
In windows (clone and related) apps, the button that looks a pair of small, overlapping rectangle at the top, right corner of the frame "maximizes" the window. That close to full-screen, but retains the frame and offer new controls, e.g., the "minimize" control.
True full-screen takes charge of either the top-to-bottom or left-to-right screen aspect (both, if your resolution is proper relative to the app's notion of its "picture" or sometimes in older apps both with stretched aspect in one or the other direction).
In reliable, modern apps, it means stretch to largest dimension while maintaining aspect ratio.