You don't give certain apps root permission, but certain apps ask for root permission by executing su binary. It's the developer of the app who decides if the app would perform some tasks with root privileges or not. In order to let apps request root privileges, you need to root your device using e.g. Magisk, SuperSU etc. (whatever works on your device).
Here the simpeline
mount -t ext4 -o rw,noatime /dev/block/mmcblk1p1 "$REALMNT"
sdcard "$REALMNT" /storage/extSdCard 1023 1023 &
And here writable ext4 on external sd
chown media_rw:media_rw /mnt/media_rw/extSdCard
chmod g+w /mnt/media_rw/extSdCard
You can change "extSdCard" like "...