Hi Franz-Josef VorspohI,
Please check below solutions for your questions.
how can I see what view is used? :
you need to activate the developer mode in odoo. you can find it in the bottom part of the General Setting menu, under the main setting menu. click to activate it.

After Activating the developer mode, you can now able to see the "Bug" symbol as per the below screenshot over the top bar. on a click on that , you can able to see the related view and and other information.

Just like the above screenshot, you can able to see the currently opened list view in the "Edit View : List" Option. On a click on that , you can able to see the wizard which contains the related view code and the external id, inherited id and etc. details.

Que 2: how to change a view. In my special case I want to remove something from the footer of the invoice.
You need to inherit the view and change it if its related to the view side.
Same as report, you can find the report template view and inherit it to change the view layout. For footer and header, odoo has the default document layout. You can change that. Please check the Inherit document of odoo or you can reference any inherit view from the addons.
Hope it will help you.