Normally you just attach an antenna to the RokuTV and perform a Channel Scan. (Settings...TV Inputs..LiveTV...Scan for Antenna Channels...Start Finding Channels).
To watch the over-the-air channels (OTA), you then select the Antenna Input from the Home Screen. (If the Antenna Input is missing, then Settings...TV Inputs.. [Look for Inputs not Used and Select LiveTV].
Not sure if I am missing something in your original question. If you think you are not getting enough channels then perform the Channel Scan again. The major networks are going to be part of your local OTA channels. (ie. ABC, NBC, Fox, CBS, etc)
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102
Being in Dana Point, your signals are likely stronger from San Diego County than from Los Angeles. At least that is what antennaweb.org seems to report. I realize Dana Point is typically seen as part of the LA region, but you're very far from the transmission towers.
Unless you can mount a directional antenna at least 30 feet high to aim it north towards Mt Wilson (near Pasadena), you won't get many LA stations. It may even be the same for SD stations, but you should do a search on antennaweb to get more information. I use that site even though I'm in urban LA, just to point my antenna correctly.
don't call me expert, just a long time Roku user that seen a lot of things