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.

In order to see current view information firstly you need to active debug mode which is done by two ways:

• Install browser extension "Odoo Debug".
• Add "?debug=1" after web in url.

After activating you will see bug symbol(debug symbol) by click you will see multiple options and by clicking of form/list/tree view you will get specific view external id inherit and apply modification to current view in your custom module.

In you special case, you need to modify template of invoice (pdf report). You need to go to the base module (unless custom  module customizing the same module installed) find report template and inherit add modification.

This link will help you to understand template customization.

• https://www.cybrosys.com/blog/how-to-create-pdf-report-odoo-13 (create new report/invoice pdf)
• https://www.cybrosys.com/blog/how-to-customize-pos-receipt-odoo-13 (customization)

As your question is unclear, this the best solution i can produce with current information.