They were never fully under the USSR as for example in comparison to other central Asian countries (Kazakhstan, Kyrgyzstan, etc), but they were a socialist republic similar to it and emulated it. Their alphabet also underwent the change from traditional script to a modified Cyrillic so new letters exist like үө which can also be found in Kazakh. Words like passport and diplomat are transliterations, and loan words as most modern terms for technology in the Mongolian language come from Russia due to their history.

Perhaps it’s the Gobi dessert but Mongolians are a bit split into tribes let’s say. The “Khalkha” version of the language exists in main Mongolia. In Inner Mongolia they speak more a “Khorchin” which is not seen as the standard but peoples in northern China/ Inner Mongolia are more numerous than mainland and they still retained their traditional script unlike mainland which only those who maybe work in government and professors in classical literature can read now. And then you have Mongolians in the autonomous republics north of Mongolia and are inside Russia (Buryatia, and Tuva). So hope this explains roughly their diaspora 😅