$node->field_file->getValue() gives me an integer (I guess the file id). How can I actually get the downloadable file uri from that?
First you need to load it with the fid :
$file = \Drupal\file\Entity\File::load($fid);
And after :
$imageSRC = file_create_url($file->getFileUri());
It is also possible to do one-liner with magic method:
Or to render as string