1) How do I choose file using File Chooser?
Your project does not work because there is no File Chooser instance on the layout, so there isn't one to activate. Place an instance off-screen for the 'Load' button to actually show a picker.
2) How can I load file from specific location without asking user for location? For example: save to Documents/GameName/savefile.json
The File System plugin can do that, but only in desktop exports. See the File System manual entry.