One doesn’t need to guess, one can read the relevant documentation. In the case of Internet standards, those are the Request For Comments (RFC) documents. Internet email is defined by the Simple Mail Transfer Protocol documentation, the most recent of which is RFC 5321:
SPF is this one:
If they are too technical for you (they are usually way too technical for me) then there’s always Wikipedia.
Setting up multiple mail servers for one domain is a matter of having multiple MX records for your domain, and giving each MX record a priority. If the highest priority server is down, senders will try to use the next highest priority MX record, and so on.
Setting up one server for multiple domains is again a matter of having multiple MX records, one per domain, and each pointing to the same server. That server will need to be configured to handle the incoming mail appropriately, of course, which has nothing to do with DNS records.
One difference between MX and SPF records is that MX records are required for internet email, and SPF records are optional. They serve two very different purposes: one is to discover where to SEND outgoing mail, and the other is to discover if it’s safe to RECEIVE incoming mail from a server that’s trying to send it to you. They are different mainly because MX is much older than SPF, and spam had not yet been invented when the MX protocol was designed, so they didn’t think there was a reason to add any kind of authentication to MX. They were having enough trouble just getting it to work at all!
Hope this helps,