To send a standard text rather than a message through Messages/iMessage on a per-message basis, just do the following on your iPhone:
Tap and hold on the message you want to send as a text. From the pop-up menu, select "Send as Text Message".
Note: Sending a message to a non-Apple device will always be sent as a text, at least unless the other user has an Apple messages compatible app running (unlikely as of 2023).
My latest blog post: Sheeple: Are they succumbing to fake news, misinformation, and propaganda. (posted 2025-09-24). My latest podcast: OJB's Podcast 2029-09-09 Kiwi Values.