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Open the app that you want to float like that.
Click the overview/background nav button at the bottom of the screen.
Long press on the app that you want to float.
A blue box will appear telling you to drag the app there to put it in the pop-up mode.
You can then drag that pop-up window around, resize, etc.
That’s the stock calendar widget. If you’re using the stock Samsung launcher, long press or pinch in on your home screen, tap the widget icon to open your widget selections, tap the calendar (there are 4 stock options), the one pictured in your post is the “list” calendar widget. Once it’s on your home screen you can resize it and make other adjustments to the widget.