No matter what, all these are "over the internet"
In other words not using the cellphone tower native voice/sms transmission
protocols, but their data connection, with the apps' own protocols "Over the Top" OTT
VoIP, Voice Over IP
There are hundreds of companies / apps / devices in this niche.
TextNow just happens to be free and noob friendly to get set up.
Do **not** settle into a Google
solution other than Google
Fi SIM card, for decades they've been chopping and changing strategies, canceling very popular services along the way, very hard to get support, for example trying to port a valued number back out.