Or you can create a disk image in one of your app qubes, place the files there, attach it to the PopOS as block device and copy the files from this disk.
In the app qube create a disk:
dd if=/dev/zero of=~/mydisk.img bs=1M count=256
mkfs.ext4 ~/mydisk.img
Mount the disk in the app qube and copy the files:
mkdir ~/mnt
sudo mount ~/mydisk.img ~/mnt
sudo chown -R user:user ~/mnt
cp ~/somefile ~/mnt
sudo umount ~/mnt
Make the disk image available in the Qubes Devices widget:
sudo losetup -f ~/mydisk.img
Attach the /home/user/mydisk.img block device to the PopOS using the Qubes Devices widget.
In the PopOS mount the attached disk, copy the files from it and unmount the disk after you finish the copy:
mkdir ~/mnt
sudo mount /dev/xvdi ~/mnt
cp ~/mnt/somefile ~/somefile
sudo umount ~/mnt
Detach the /home/user/mydisk.img block device from PopOS so you can mount it in app qube and use it there again.