Full digestion of food after eating takes between 6 hours to 8 hours for the food to be digested and then be pooped out through your bowels.
The stomach digests the food and then sends the waste through your intestines and bowels where the waste product of the food is then pooped out eventually.
Your stomach acid digests the food.
Food does still digest when laying down but the food when lying down doesn't digest as good or fast as it does when sitting up or standing up.
Our bodies were made to digest the food we eat while we are in an upright position and laying down right after eating can lead to digestion issues and indigestion.
So it's best to wait 30 minutes to an hour after eating before laying down to allow your body to digest food properly.
If you lay down too soon after eating then you can have heartburn and indigestion issues.
But your body still does digest some food while you're laying down but your body should be in the upright position to get most of the food digested before laying down.