I think one of the most impactful choices in my last build was choosing a fast SSD. Not all SSDs are the same! Nowadays you can get NVMe drives that operate over PCIe instead of SATA which provide much higher throughput.

You can either get an M.2 form factor that plugs into a special socket on the motherboard and takes up minimal space, or a PCI card that plugs into the same type of slot as a graphics card. (Note that some M.2 drives / sockets are SATA, not NVMe, so watch out for that distinction.)

There is also some difference between NVMe implementations depending on which PCIe version they support. And you'll want a motherboard that implements the same PCIe version. This applies to both M.2 and PCIe SSDs.

This stuff might be old enough that you've already encountered it. But it was new to me when I built my last PC in 2020. Other than that building was pretty much as I remembered from previous decades.