One of the key areas where people modify their QGIS setup is which panels are used and where they are placed. The key panels in QGIS are the layer panel (1), and the browser panel (2), but also the layer styling panel (3) is commonly used
Here Panel 1 and 2 are docked, i.e. attached to the edge of the QGIS window, while panel 3 is floating Panels can, be closed by clicking the x in the top right corner (standard windows behavior) when a panel is closed it can be opened again from the view menu (view ->panels) and set a tick mark in front of the panel you wish to open
Pannels can be moved and stacked on top of each other; to see this section of my video on QGIS UI issued.