Changing Theme – editing Frontpage with WordPress

This question cannot be properly answered without a URL to your website, or more details on the “theme that was created by an agency”.

My first comment would be to generate a Child theme for your website, if that has not been done already. Do not make changes to the parent theme.

Also – the agency that delivered your website didn’t provide any documentation on the structure of your site?

I don′t have a documentation. But I have all the files from it. It′s a lot of different PHP files and others. They build the Homepage. I could edit them but my skills are not good enough to do it well. So I tried to use some Plugins to use the Wordrpess editor like “Gutenberg”, “Elementor” or others to add Blocks to it. But this is not possilbe. So maybe there is a plugin which can do this. I just worry there is no way for it but change the PHP files by hand.

You are correct.

You can use a plugin like this to see the files that are being loaded on a page:

which template file – WordPress plugin | www.ads-software.com English (Canada)

or

Quickly find which WordPress template file is currently being used – Meks (mekshq.com)

It sounds like this is a Classic theme, and not a Block theme? Another option would be for you to switch to a block theme, which enables you to make changes to any page layout using blocks. This article will help clarify the differences for you:
https://www.pootlepress.com/2023/04/wordpress-block-themes-vs-classic-themes-a-beginners-guide/