I thought SMS apps just split the text in 160 character chunks, and merge them on the receiving side. Isn't that how it works?
Yup, the parent comment is incorrect. SMS longer than 160 characters (in GSM 03.38 encoding) are split into multiple parts, each a maximum of 153 characters long, 7 characters are used to glue the parts together.
I believe it depends the implementation. I recall having a phone that would explicitly indicate a switch to MMS when sending longer messages.
I too recall that. I suspect it was a boon for the carriers, eating into your MMS quota. At the time though, my plan was unlimited SMS/MMS, so it didn't really matter to me.