Yup, transistors especially have given us major breakthroughs in the ability to step up/step down DC.

Also, there have been huge breakthroughs in High-Voltage DC: https://en.wikipedia.org/wiki/High-voltage_direct_current

At certain huge (grid) scales they have found that AC and DC swap "efficiencies" again and we're increasingly starting to see current flows as DC-AC-DC "sandwiches" with DC used by the majority of consumer electronics and DC used for extremely high scale grid transport, and AC still useful in the mid-range transport.

Yes, this is true. To step up/down AC voltages, you only need a transformer, a pair of coiled wire. This is very simple tech.

To step up/down DC. There are ways to do it with solid state electronics. One of the ways I’ve seen is to transform the dc to ac internally, change the voltage, and convert and output DC.