I am wondering what the standard tool is for sending and receiving SMS and MMS on a device that does not have a SIM card in it.
Is there some tool that can do it natively? Is there a specific carrier that is commonly used for this? Is there some sort of selfhosted service that bridges it to email, and if so do I need to put a SIM card in my server?
Bonus points if I can do it within Emacs.
Depends on what you are trying achieve.
You can send sms via foss no problem as long as you know what carrier the recipient uses. All of the carriers seem to have email bridges to the sms network. Receiving sms is another question entirely. To receive sms, the network needs to know where to send the message. There are commercial platforms that can link you into the sms network.
Another option is KDEConnect which can link your Android device to your computer and you can send and receive sms that way.
https://simpletexting.com/blog/how-to-text-from-computer/