Hello @m.gurkangurer,

Regarding your first question if you have already set up a custom homepage and now are trying to show separate blog page to display all posts from different categories then please try following the steps below.

1. Create a new separate page e.g. Blog from Page menu of Dashboard.
2. Now, go to menu Settings -> Readings and choose second radio option on Front page displays label. Here configure
Front page : Select page from the options to display it as main homepage
Posts page : Select page from the options to display it as a blog page e.g. Blog page here.

3. Finally Save and view your blog page that should display all posts from different categories as a blog.

Hope these instruction would address your first query.

And for adding thumbnail in the blog lists, first of all there should be featured of each of the posts that are listed in the blog lists page. After that, go to Themes Options -> Layout Options where choose ‘Excerpt and Thumbnail’ under ‘Archive Layout’. It will show thumbnail along with the post excerpt.

Hope that helps resolving your issue.

Besides, if you still have some confusion somewhere then kindly post in query with your site URL so that we could specifically answer them.