> I know imperial was invented by the British, but it's largely just the US left using it

According to wikipedia, the US do not use the "Imperial System" but the "United States customary units" and they are not exactly equivalent, although they both derive from the "English units ".¹

Also, from the article: "The international foot is exactly 0.3048 of a meter". And it seems the international yard and pound are defined using the SI units as the reference.² (Edit: So by transitivity they also are defined using caesium-133 atom's radiation, light speed in vacuum and Planck constant.)

¹ https://en.wikipedia.org/wiki/Comparison_of_the_imperial_and...

² https://en.wikipedia.org/wiki/International_yard_and_pound