Most are DIY, anything that is low wattage can be turned fanless if you buy some parts and build it yourself. But OP was daft with his answer. As someone who doesn't like the Apple ecosystem and only uses PC, I can only admit defeat when it comes to power efficiency. There is no way you could make a true fanless system run as well as Apple's efficient SOCs. PC parts that are as powerful generate a lot more heat and even with the best fanless cases and radiators there is only so much you can do to dissipate heat.

Maybe industrial applications for dust-heavy environments or avoiding movable parts in a general make up most of the fanless niche, but some manufacturers offer desktop-like system as well. Desktop-like because extensibility is limited: these systems are typically built around mini-ITX boards, and the passive cooling solution may have limited options for cooling SSDs. Sustained CPU performance can be quite good, particularly with the Intel T series or the AMD equivalent.

DIY is a bit hit-or-miss because if you want a truly silent system, you need select components for absence of coil whine, and I haven't found a good way to do that yet.

I highly recommend a NUC with a fanless case from Akasa.