is there a way to merge different QGIS project-files? For example, i have one project with water supply data. The project organizes the relations of the tables, the styling etc. In another project, the topic gas pipes is organized. One customer wants this topics combined in one project. Is it possible to merge those two existing ones?
Option 1 should suit your case better as you could get all layers group in one go.
PS: if you use filesystem for some of your layers and not a database, you may have to change/copy your files for layers and also change data source to make the project "portable" relatively to the project path after applying the recipe from my answer.
Open your first project, then go to "Layer" > "Embed Layers and Groups...", select the project you want to borrow/import layers. Select the layers/group you want to import and it's fine.
You can also open two QGIS instances and copy from one project to another (right click on layers/group layer tree to choose "Copy layer" or "Copy group") and paste in the layer tree from the other project.
In the layer tree, right click on layer, choose "Export", then "Save As Layer Definition File..." from the project you want the layer(s). Then import it in another project with "Layer" > "Add from Layer Definition File..."
In the latest QGIS 3.14, you can now drag and drop layers between projects of different QGIS instances e.g https://changelog.qgis.org/en/qgis/version/3.14/#feature-allow-dragndrop-layer-across-several-qgis-
If you know other ways, curious to know how.