Thank You Hadron for a very good explanation of how texting works. It is certainly complicated. I get confused between MMS, RCS, SMS and now my phone wants to send something called "RCS Chat". From what I can find out, RCS is quite an improvement in security over SMS.
In the last paragraph, you mentioned not being sure about no way to send without attachment. I apologize, but I explained very poorly. Here is another try:
A person named "Louis" sent me a text message with a photo. ("Attachment")
It is still in my inbox 50 days later.
I wanted to send Louis a text. My new text got sent to Louis with the one he had sent to me 50 days ago, including the photo.
It seems to send a new text with to someone who already has a text in my inbox- with no old texts attached- all texts from that person should be deleted from the inbox before composing & sending the new text. Best I've got.