Mortar shells are much easier to make than howitzer or cannon shells. (Much) Lower working pressures.
But I suspect that what the gentleman is holding is NOT a mortar shell, but a howitzer shell. Dumb lazy fuck journos and/or machine translation.
One - it does NOT look like one. The shape is wrong, and those bands are IMO too far back, suggestive of being
driving and not
obduration bands.
Two - in Russian (like in German) mortar (mortira/moerser) can be used for howitzer (retaining oldest meaning of mortar, i.e. a special form of howitzer) . What is commonly called a mortar in other languages is "minethrower" (minamiot). Ukrainian could be same.
But I could be wrong and he actualy is holding a shell for a 12cm or so
fancy mortar