As I understand it. The specification for them comes from the ECMAScript standard (Language Specification). The actual implementation details is engine/vendor-specific. Which is how something like .sort() can have different behavior depending on the browser.
When JavaScript was wanting to be standardised, Netscape and Microsoft couldn’t agree with each other about things, so an external unbiased source of standardisation was sought.
It fell into the hands of ECMA, thus giving us the name of ECMAScript.