The word acronym typically applies when the resulting thing can be read as a word; for example, radar comes from “radio detection and ranging” and scuba comes from “self-contained underwater breathing apparatus.” The word initialism only applies when the resulting thing is read as an abbreviation; for example DIY, which comes from “do it yourself,” is pronounced by saying the names of the letters.
NASA, not NAySA, SCUBA, not SCUBbA, LASER, not LASsER. Many such cases.
ATM is not an acronym, though, just an initialism, so the letters are pronounced individually.
All initialisms are acronyms
https://www.merriam-webster.com/dictionary/acronym
From that link:
Right, typically, not exclusively.
It’s ireelevant anyway since the english pronounciation is so inconsistent.