how can I make GIF over a static image?

[rich2005]

Gimp is a complex application, the best learning method is start off with simple operations and just go through all the various tools and modes to understand how they work.

Absolutely no idea what you are doing.
As previous post the foreground select tool is described here: https://docs.gimp.org/2.10/en/gimp-tool-...elect.html

This definitely last post from me.

Some options for removing backgrounds https://youtu.be/T3yEuiWwPAc duration 11 minutes
Simple method using eraser tool and fuzzy select
How the foreground select works
Using the G'mic plugin interactive foreground select
Assuming some day you get the 96 layers editied adding a new background.