Boycott US

Non-American CPUs are very rare. NXP and Infineon are European chip makers, but the only device with an NXP SoC that I'm aware of right now would be the Purism Librem 5. An almost 7 year old, absolutely overpriced, nearly unusable, insecure Linux phone.

We're a little more lucky when it comes to storage. Goodram is flash memory manufacturer from Poland. Intenso is based in Germany. G.Skill and ADATA are Taiwanese. Toshiba and Kioxia are Japanese, SK Hynix and Samsung are South Korean.
(TrekStor is also German, but I'm not sure if they make the chips themselves)

Most mainboard manufacturers, such as MSI, AsRock, ASUS, Gigabyte and Biostar are Taiwanese.

Then there's also the software to think about. You can certainly use Linux instead of Windows but even then, lots of people will also end up using Google, Steam or Discord.

rockchip based SBCs are powerful enough for general-purpose use. Some of the companies behind them are american like PINE64 but most arent. There's a promising board called the orion o6 coming out of china but its still really young in its development and support lifecycle and its hard to source.

Open source Risc-V CPUs are starting to become a thing, but probably a few years before they're widespread.

Really wish Nortel was still around for networking. No idea for good business grade switches and access points.

Is it really American computer hardware if it’s all manufactured outside of the US?

Moore Threads is a domestic Chinese company that seems to be trying to compete with NVIDIA, AMD. Their GPUs look comparable to GTX 10xx series cards. Wish I could get ahold of one to try it out. I hear the drivers are pretty crappy but have been getting better fairly recently. Fortunately, most parts are fundamentally based in Taiwan due to TSMC so you might have to research clients of TSMC to find alternatives. CPUs are a pain point. INTEL or AMD. ARM is cool but until RISC really takes off, your going to be tied to at least one or two american companies.

Nvidia/AMD/Intel/Apple/Qualcomm/Broadcom < Matrox/Imagination/SiPearl/Arm/Nordic Semiconductor

Plenty of Taiwanese based companies for PC parts (ASRock, ASUS, BenQ); though I don't know if any of them have CPUs, specifically, and the GPUs are still based on specs from nVidia or AMD so IDK how much better that is. There's also Chinese based parts, though I have to imagine that's not much better than US ones, for the same reasons you want to avoid American parts.