for the first part, the same way the pros do it with very thin copper clad boards. you start with a double side board and bond single sided material on to build the layers.

interconnects between layers are done with vias, with this process I can't use blind vias only ones that go thou the board.

as for the tin plating in the holes their are three ways listed best to worst

board houses use and electro chemical bath that plates all exposed copper and bridges the gaps between the holes and interconnecting the inner layers. works great but the chemicals are nasty and have a short working life. I don't use this process.

second is the auto contact from LPFK it's an attachment for the milling machine that coats the inside of all the holes with a special solder paste that is baked before soldering. it works good but if you remove the part after soldering it damages the plate thou also the paste only keeps for a few weeks in the crisper.

last and I hate using these are copper rivets in the holes they take a min or two each to install, and can cause intermittent contacts after they start to corrode.The workaround to this is to solder both sides of the board

Also it helps if you can keep the auto router from placing the vias under the parts, for some reason it just loves to do that.

I also have Sweety to contend with while trying to work.