How are plated through holes (PTH) made?
In general, PTHs are done in two basic steps.

1) A very thin (1 micron thick) conductive layer is deposited on the hole walls.

2) A thicker layer of copper is electroplated over the thin layer.

The thin layer is the tricky part. Classically, this is done by depositing electroless copper on the board. All sorts of different chemicals can be used, although the most common involves copper sulphate, sulphuric and hydrochloric acid, sodium hydroxide, formaldehyde, and several other chemicals in small quantityl, including a catalyst solution containing palladium and tin. Google "electroless copper plating solution" for sources. The process is not very DIY-friendly, but you can buy the solutions. A special problem is disposing of used solutions, and maintaining the solution chemistries over any sort of long term is a challenge for hobbyists.

Recently, the use of conductive inks, usually containing graphite, has become popular in the industry. A big advantage of these inks is that finicky control of process chemistry is unnecessary.

There is a third technique which involves depositing a copper-bearing chemical on the hole walls, then pyrolyzing the material to leave behind a copper film. This is attractive for hobbyists, but the material needed can also be used in methamphetamine production, so it is a List I precursor and buying it will bring you to the attention of the Feds, who have no sense of humor or proportion on the subject.