As far as I'm aware, you cannot import custom ringtones you purchased outside of the iTunes store without plugging it into your Mac.
Still. In 2020.
How does a person put these on their iPhone without a port? https://twitter.com/joshuaginter/status/1320734401102286855
@joshuaginter There’s also the folks using HDMI output. I regularly have guest speakers show up with just an iPhone.