It's completely closed (not even source available) but there's unlicensed software implementations available from third parties. E.g. libmbe.

AMBE in most equipment was provided in a physical chip for protection however some firmware implementations existed in hardware from big players and they were reverse-engineered.

Most of the Chinese B-brand (AliExpress stuff) radios use this reverse-engineered software implementation. They're not licensed but a Chinese company pretends to license this on behalf of DVSI though it's totally illegal. The A-brands like hytera use the official one of course.

See the popup warning on this page: https://www.dvsinc.com/soft_products/software.shtml

Since the closed nature of this codec goes counter to ham radio principles some open source codecs were also developed. The best example is codec2.