Portable Project

You will need a portable version of your QGIS project file (.qgs). Portable means that all paths are relative and datasets are reachable from the device.

Check that Project --> Project Properties --> General --> Save paths{.interpreted-text role="menuselection"} is set to \"Relative\" and that all required data files are in the same folder like the .qgs file or a subfolder of it.

For increased productivity, we suggest having a look at the QFieldSync plugin.