Depending on how you apply the Velcro, you can close the windows.
If you put the Velcro on the sides of the window doors, you won't see them, but then it will be almost impossible to close the windows (because the Velcro + the tarpaulin will be too thick for most windows or the closing mechanism will get in the way). The pictures here on Galaxus are the variant where you can no longer close with the tarpaulin.

The Velcro should be on the front side of the window/window frame. Then it works with the closing.
It's a bit difficult to explain without pictures, but I hope it was understandable enough?