If an oil galley plug was loose or missing there wouldn’t be any oil pressure, and there would be a huge oil leak.
There are two plugs, one at the front of the block and one at the rear, to plug the holes where the main oil gallery is drilled from the front to the back of the cylinder block. The one on the front gets covered by the timing chain cover at the front. The other is on the back of the block, and is directly in front of the forward side of the flywheel (or flex plat if the engine is mounted to an automatic).
There are no other plugs in oil passages in the cylinder block.
The crankshaft has plugs where oil passages get drilled to carry oil from the main bearing journals to the rod bearing journals. In my experience not many machine shops remove them when performing machine work on crankshafts.
Oil is picked up from the crankcase by the oil pump, sent through the forward passage to the oil filter, then goes through the reward passage, some going to the passage that feeds the crankshaft main bearings (and ultimately the rod bearings and some squirts through a small opening on the upper side of the big end of each connecting rod to lubricate the cylinder walls and pistons) with some bypassed to pressurize the cam chain tensioner, and some going to the cylinder head to lubricate the cam journals and the valvetrain.
The L series lubrication diagram: