All the other elements. The universe is mostly hydrogen and helium.

But that means that every atom besides hydrogen, at some point, has undergone this process.

If we disregard the fact that by mass, most of the universe is comprised of gas clouds (since they are kinda in the process of becoming stars) we are left with stars and other bodies such as planets and asteroids and moons. If we take our solar system as the example, over 99% of the mass therein is contained within the sun.

The conclusion from this is that, although the overall composition of the sun isn't rapidly turning into helium or anything, there's about 600 million tons of hydrogen in the sun undergoing fusion every second, producing nearly that same weight of helium.

Is that significant in the grand scheme of things when discussing the universe? Absolutely not. But it's still an absolute fuckload of hydrogen changing into helium every moment across the span of the 4.6 billion years of existence of the sun.

I think as a thought experiment in the immense amounts of helium being produced this is worthwhile to consider. If you would argue that I'm defining away the problem or that I've shifted the goalposts so far as to be discussing something only tangentially related to the first point, I wouldn't disagree with you on that. But for the average person, the quantities of hydrogen becoming helium in our sun alone is absolutely mind-boggling.