Having updated your iPad, you cannot restore the earlier version of iPadOS.
As you have no doubt discovered, update to iPadOS15 introduced the ability to add Widgets to your Home Screens - some being added to your first Home Screen, by default, during the initial update to iPadOS 15.x.
Widgets placed on a Home Screen now individually occupy one or more elements of a 4x6 icon grid. Home Screens without Widgets retain the familiar 5x6 icon grid with which you may be familiar. With the grid accommodating fewer icons when Widgets are present, other App and Folder icons may be displaced to the next Home Screen - and this may have a ripple-effect where Home Screens icon grids are fully populated.