My goal is to collect data on trees where the same trees are visited multiple times, using QField. While each tree will have a unique ID, we have to use the GPS-coordinates of the trees plotted on a map to locate them. The spatial point layer we have now are basically the locations of those trees. We have to revisit all of these trees over a period of time, and collect data. In ArcGIS Collector, there is the function 'collect here', but that adds another point with the exact same coordinates. We just want to add another row of data, so that multiple rows correspond to a single location. In other words, we do not want to keep adding points, because the locations do not change, and do not want to overwrite previously recorded data. After a bit of searching, I know this is called one to many relationships, but I have not found out how to create this relationship with an existing spatial layer with QField. I have not been able to find any sources or tutorials on how to do that with an existing point layer. So in short, we want to use QField so what we can locate our trees. This should be straightforward as we can just load a map of all the tree locations on our devices. Next, we also want to relate the collected data to the specific individual trees without adding another spatial point to our dataset.
You need to first create a table that will store your observation, this table should have a foreign key field that will store the tree ID (and of course all other relevant field for your survey).
Then when you set up your project in QGIS you need to set the 1-N relationship between the tree layer and table and configure the edit widget to be able to use it in QField.
I suggest you to read the documentation on the below link and try to set it up, then if you encounter problems you could edit this question or open a new one to solve your specific problem