When the modular SIG P320 and P365 were launched they touted a chassis-like system for the fire control groups. Others like Grand Power had been using this construction style for years to improve longevity and trigger feel consistency, but SIG managed to make the FCU the serialized part recognized by the ATF as the legal “firearm.”
This opened up a world of possibilities. Other manufacturers like SCCY and Honor Guard did the same, but the modularity never went much beyond the same grip modules being offered in different colors.