When German companies post job ads, they usually only do so on official job portals. They also use commercially operated online job boards. You can generally create search filters to look very specifically for your new job in the desired location. There are job portals with offers from across Germany, as well as regional job portals that are focused on a specific region or a federal state such as Rhineland-Palatinate. The best thing to do is simply try both.
We are unfortunately not able to name specific providers here for reasons of competition law.
Have you already found a few companies that you are considering? In that case you should absolutely explore these firms' websites. More and more companies are including current job openings on their own websites. You generally find these offers in the “Job openings”, “Current/News” or “Career” sections. You can also find interesting offers on social media.