It’s depends on the specific engine, but you essentially tear it down then replace or machine parts back to high quality. For example, cylinders and walls may

It’s depends on the specific engine, but you essentially tear it down then replace or machine parts back to high quality. For example, cylinders and walls may develop too much gap over the years. You can machine the walls out a bit then slide a sleeve in to get it back to original sizes. Lots of manual work, but cheaper than a new engine. You might also replace a bunch of other components along the way. The key thing is the engine block is generally still sound, even if other parts have warm out a bit.