I have some blog pages and regular pages where I have some products/ads shown now and then after some paragrahs in the text.
Currently, when I need to change these products/ads on the page - I have to manually edit all posts and pages.
I know I can't insert liquid code in the editor - but is there any other way to automatically include a snippet or other type of content into the editor - so I can just edit the stuff one place - and then it's updated in every page / blog where it's included?
Absolutely. You can add your code into templates/article.liquid and templates/page.liquid. These are the blueprints for all pages and blog articles.