I think at least with the Pi 4, you can limit the CPU clock by editing a config file, which has a power limiting effect.

If you set the scheduler to powersave, it downclocks all cores and saves power that way.

Yes, there is a low power mode. And you're forced to use that if you startup with an "under"-powered supply.

Jeff Geerling on YT has a great in-depth review of these options in his latest video.

I read a lot of Jeff's blog, but his vlogs put me to sleep. He isn't charismatic and makes the worst jokes. People forget that a visual medium is visual. He channel is an example of great information that is ruined by the visual medium.