on today's modern systems framerate has been complicated by the introduction of frame generation technology like DLSS and FSR. If a user has a capable GPU and has frame generation turned on then most software will see the FPS number as including the generated frames. This is correct if you think about just the smoothness of the video and how frequently the monitor is updated. However, this is incorrect if you care about actual game frames, which do other work like process input, handle network updates, perform collision detection for models and projectiles, etc. Frame generation can't help with things like input latency that matter to competitive gamers, but it can make things look visually smoother on todays high refresh rate monitors.

The Steam Performance monitor will detect frame generation technology and break down both the DLSS/FSR Frame Gen including FPS and the actual game FPS over 1 second intervals. Further, the overlay will show the minimum and maximum single game frame performance within those one second intervals. If you see a single FPS XX number plus the ↓Max↑Min, then your game is not actively using frame generation. If you see a DLSS/FSR/FG number followed by FPS XX ↓Min↑Max, then your game is using frame generation actively and you get both that display frame rate including generated frames and the actual game frame rate. Note that frame generation enabled games will commonly switch frame generation off in menus and cutscenes and that this is normal and correct behavior to see.

One question is, how should one understand the GPU vram usage, when it is reported as 16.0/15.7 GB?

IIRC, using more vram than what's on the card isn't a show-stopper, just slow. The parts that don't fit on the card just lay in RAM and are swapped back on card when needed. This is fairly slow and comes with a performance penalty, which is why the numbers are shown as red on there.

as for why it's not showing full 16 GB as being the max? ... no clue, probably the card needs some vram for it's own operation/general framebuffer (or whatever is the term) for displaying eg. the os, not just the game. But I'm just guessing.

The one on the deck is based on Mangohud. This one seems to be custom made