Volvos, and especially their engines, are, like Rolls Royce, built "up to a standard, not down to a price"

My understanding is that the original B4 four cylinder (Volvo's first) in the earliest PV444s was derived from the 1089cc Fiat 508C engine of 1936. The Volvo engineers basically copied the best OHV 4 cylinder design out there at the time, enlarged it slightly (to 1400cc) and strengthened it considerably so that it would last indefinitely. I'm not familiar with either of these engines, but a B16 (evolved from the B4) does bear considerable resemblance to a Fiat 1100/103 (evolved from the 508C) of the late 50s and early 60s, though, of course, the Volvo is bigger and stronger.

Previous Volvo engines, all sixes, were derived from American practice (Plymouth and Pontiac come to mind)

The B18 was developed from half of a V8 that was intended for the stillborn "Philip" project (a big sedan that resembled a Kaiser---only one was ever built) of the mid 50s. It was a state of the art design for the time, with a short stroke, massive crankshaft (which remained unchanged when the extra four cylinders were lopped off) and a stiff lightweight block. This grew into the B20 a few years later. The OHC red blocks (B21, B23, B230) are a further modification of this rugged bottom end, again with Fiat influence. The system of inverted bucket tappets with large diameter adjustment shims running directly against the cam was developed by Fiat in the mid 60s, when they converted their OHV 124 engine (that looks a lot like a smaller B18 with an aluminum head) to DOHC. I believe Volvo actually took out a license to use parts of this design. The adjusting shims are identical and interchangeable between Volvo and Fiat. While adjusting valves is a bit of a hassle, this is probably the most trouble free design of cam and cam followers ever used on a production car. I've never seen anything go wrong with one, whatever the mileage.

Yes, the high quality cast iron used in the blocks is a factor in the longevity of these engines, which far surpasses that of most Japanese cars. Most Toyotas and Hondas are usually good for 250-300K (though I've known several people who had Toyota pickups with the 22RE, all of which needed a cam, rocker arms, a valve job, and a timing chain and sprockets before 200K, sometimes well before) given reasonable maintenance, but B230s can go way further than that. I know of one with 360K on it, all original and untouched. (never even had the valves adjusted) There was an 88 744 featured in Rolling (the Volvo club magazine) a few months ago. It had over 800K on the original engine, and the rest of the car looked like new. The only Japanese car I've seen that can come close was an OHV Subaru (early 80s) with over 600K on it, by which time the body was getting pretty rough around the edges.

Interesting trivia bit about Swedish steel: Over 90% of all musical strings are made of it, wherever they are manufactured.