The new Canon mirrorless bodies seem to allow one to plug in a portable HDMI monitor (perhaps USB powered) into the micro-HDMI socket, and use that as a large viewfinder. Unfortunately, I don’t think Canon’s DSLRs allow this, even in ‘Live view’ mode. At least, I’ve never got it to work, even though they some of the DSLRs do have a (seemingly rather useless) micro-HDMI socket.
There is an official Canon EOS app, which allows some newer DSLRs to be controlled from a phone or tablet. It’s a really horrible hacky app which takes over your phone’s Wifi settings, and uses Wifi to communicate with the camera. But on the few occasions when I’ve tried it, it has worked. It gives a viewfinder image on the phone screen and lets you control some of the cameras settings through the phone.