@esti
Liquid gasket sealings are a source of Silicone, too. The Al which you observe together with dirt seem to clearly come from the bearings.
Sand is used as die cast material during cylinder block /cylinder head production.
It is rather hard to clean a new block completely from sand which is situated in oil channels, etc.
You are right with your coolant observations. Silicone in coolants is a classic chemistry for coolants. New longlife coolants are most of the time silicone free and have Kalium instead of Natrium.
The copper may come from welded oil cooler fittings, sometimes bearings, etc.