This is entirely normal behaviour for your iPad Air4.
The slim bar that appears at the bottom of the screen is a feature of the 2020 iPad Air4 and 2018/2020/2021 models of iPad Pro - all of which do not have a physical Home Button. If you have previously used any other model of iPad, this bar will not been seen.
The bar to which you refer (black on a light background, white on a dark background) is a “handle” that is used to initiate some gestures and functions - similar to that which you see at top-right of your iPad lock screen that indicates access to the Control Centre.
Whilst early observations may suggest otherwise, this handle plays no part in the upward-swipe gesture from the bottom of the screen. You can demonstrate this by swiping-up from any part of the bottom edge of the screen. However, if you touch and hold the bar, you can drag the bar left or right to switch between recently used Apps.
You are not the first, and certainly won’t be the last, to notice that the bar at the bottom of the screen is persistent; for most Apps, the bar does not disappear. This is not a fault, but appears to be a deliberate decision on the part of the User Interface designers for iPad models that lack a physical Home Button. The bar can be suppressed by Apps that are specifically coded to do so, however, experience shows that Apps with this capability are very much in the minority. This feature is not user-configurable.
Apple do invite constructive comments and feature requests via their product feedback pages. If you have feedback on this feature, this may be the most appropriate channel to make your own feelings known:
Feedback - iPad - Apple
