You can’t mark them as spam easily. The phones don’t have built in features for that. You have to go through a bunch of manual steps that Apple and Google should really make a one click process. See my other comment here:
What does the “Report Junk” button do then on iOS in Messages app? I use that for unknown spam sms, and was under the impression that it sends it to Apple and/or my carrier for processing / feeding some ML classification funnnel.
Must be phone specific. I have a (not so fancy) Android phone and I've never gotten a call or a text that I couldn't block and mark as spam in a single step.