I don't know about "most smartphones," but the FM bits were commonly a built-in with Qualcomm and some other chipsets for a coon's age.

Even the OG Motorola Droid, of 2009, had the parts inside for an FM radio. They just weren't connected to an antenna and the default software stack didn't include any access to those parts at all.

Flagship Qualcomm-equipped Samsung phones up to the S20 or so tended to be ~similar: The FM tuner guts were usually in there, but they often weren't available to use, and even then they sometimes lacked an antenna.

This is not intended to be an exhaustive list. :)

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In my time, I've enabled FM radio on a few different phones that weren't advertised to have that function, sometimes with root-only hacks from grimy corners of the internet, and sometimes by just installing some software that just happened to have the right hooks.