Nothing fancy is needed when the supply voltages are low, and common ground.
Level shifters typically use a pair of NMOS driven complementary, pulling on a cross-coupled pair of pull-ups (PMOS). This gets full CMOS voltage levels, with consistent timing and low supply consumption.
That is, there are two transistors pulling up, one gate connected to the other drain and vice versa, so one is pulling up strong while the other is off; yank down on the pulling-up one, and the other turns on and pulls up, then the first turns off. It's bistable, and only draws current when changing state.
Or they may simply burn quiescent current on a more analog (CCS into floating resistor?) approach. Some level shifters draw relatively high currents (fractional mA) -- the transistors in the middle of the circuit are quite small (100s nm) so it doesn't seem like much on the board level, but it's decidedly higher than proper CMOS.
Schmitt trigger circuits can do a similar thing, they might not draw much current most of the time but for intermediate input voltages, they can draw quite high currents (low mA), something to watch out for on low-power circuits.
Gate drive ICs almost uniformly use an input circuit with an NMOS, into resistor or CCS pullup, into a Schmitt trigger. The NMOS gives a TTL compatible input threshold. The pullup accounts for much or all the quiscent current drawn by the things, annoying if you want a low power gate driver.
Bootstrap gate drivers do it up to 600V with pulsed CCS level shifting. The high side stays latched in one state or the other, and receives pulses through a high voltage transistor straddling the barrier. The downside to this method is, it ignores inputs if the high side dips below ground (the HV transistor can't "transist" in that direction, indeed it acts as a diode instead), and if the latch's state gets corrupted, the whole thing probably destroys itself. Fortunately, being on-chip, it has pretty good immunity, and this isn't a problem in practice. There's also additional conditioning circuitry to ensure consistent operation during dips or rapid swings.
Still others do it by differential capacitance, through small on-chip capacitors; these can even offer full (galvanic) isolation, like TI's ISO family. Others use onboard transformers(!), like Analog's ADUM series (was the first I think, and everyone else has them now as well). Transformers aren't easy to make this small; they typically run at a few 100 MHz, and have poor efficiency (~50%). Still, it's enough for a watt or so, and that's enough for a lot of common applications, like isolated serial ports, or small gate drivers.
Tim