Usually audio equalizers are an integral part of specific apps, like music players, or can be baked into the device's core firmware, and each manufacturer tends to do their own thing. And then there's things like Dolby, DTS, Super Sound, etc. that device and app makers license and add as well.
There are third-party equalizer apps, which are supposed to add their equalization to the sound and maybe can be set the same or maybe even synced across multiple devices, but I don't think they can control other app or device system equalizers and effects though.
