I’m a bit confused about your question
For your Plex Media Server to be able to watch / record OTA channels, you need a Tuner to make those available. Based on the list of supported tuners, Plex is not limited to OTA tuners but can also deal with some cable tuners – however it won’t deal with encrypted channels.
Plex does not integrate with the tv networks’ internet live streams to make those available.
There’s a bunch of free internet channels available via the online media sources of Plex – however you cannot record those.