First one might have been the G4 Cube.
The Cube is an interesting case, and I'm not sure that there are a ton of parallels to a modern SFF computer. I say that as something a Cube enthusiast(yes, there is such a thing-remember I'm a Mac collector among other things, and have 5 total ranging from bone stock to pretty heavily upgraded).
For one thing, they're actually surprisingly large. The desk footprint is about the same as a Mac Mini, but they're tall. They're actually almost exactly the same dimensions as the Mac Pro 6,1, aka the "Trash Can." There are other parallels between the two.
The second thing is that the Cube actually uses full blown desktop components. The CPU cards are pretty freely interchangeable between graphite/Quicksilver towers. They use the same 300 pin rectangular "socket" for lack of a better term and mount by standoffs in the same location. There are some caveats in that-the Cube VRM module can't handle the factory dual CPU cards, and some of them are just too big or have too much overhang to fit(plus the Cube heatsink, which is a gigantic aluminum slab, won't necessarily fit all dual CPU cards).
You also have, tucked into the "core", a standard 3.5" ATA hard drive. The optical drive is more laptop-like, but is actually kind of sort of shared with the slot loading iMac G3(I don't think they ever had a CD-RW option, which was BTO on the Cube). The memory is standard PC-100 168 pin DIMMs.
It had a standard 2x AGP slot also...or really not standard since it remapped some unused pins plus added an extra to support ADC(Apple Display Connector-powered the monitor, provided 2 USB ports, and allowed system power on through the monitor via one oversized DVI-like cable). The faceplate was also unique, but cards, both proper Mac cards and some flashed PC cards can be made to work-the biggest limits are the physical size of the card and whether or not the VRM can support them. Most of mine have Geforce 2MX cards, one has a flashed Geforce 6200, and one has a prized Geforce 3Ti.
The first "true" SFF Mac I'm thinking of is the Mini. They used the same 7447A CPU used in the contemporary iBooks and PowerBooks, plus the mobile version of the Radeon 9200 also used in some of those computers. It's all rounded out with a 2.5" ATA HDD and a slot loading drive that's also interchangeable with the iBook/PowerBook. The PowerPC Minis weren't great computers. The Intel ones did bump up to a desktop grade C2D CPU, but the early ones used the terrible GMA 950 Intel GPU. They also had a 2.5" SATA drive and SO-DIMMs.
But yes, coming forward to 2021, SFF PCs, whether made by Apple or someone else, do mostly use laptop components. Many of them also rely on what are now very good integrated GPUs. That makes them inherently less power hungry than desktop PCs, although they will generally outperform a laptop with the same specs just because they can move more air and get rid of heat better than a laptop can. At least with modern Intel CPUs, the advertised clock frequency is kind of a joke since most can go way over it if needed and won't overheat doing so, while they will also downclock to cool down if needed, or alternatively will do so to save power. The new M1 Macs publish a clock frequency somewhere, but you have to really go out of your way to find it(it's not front and center like with x86 CPUs) and really can run at any speed they need to get the job done as long as the temperature stays under control(which it does most of the time-after 10 years of using Macs I can't tell you how nice it is to have a computer with the CPU die running at 30-50ºC rather than a "norm" of 70-80º, and temps well up over 90º not uncommon if hammering it).
