http://cameradecision.com/compare/Canon-EOS-77D-vs-Canon-EOS-70Dhttps://www.imaging-resource.com/cameras/canon/70d/vs/canon/77d/https://www.quora.com/Which-is-better-and-why-the-Canon-77D-or-the-70Dhttps://versus.com/en/canon-eos-70d-vs-canon-eos-77dhttps://www.apotelyt.com/compare-camera/canon-77d-vs-canon-70dhttps://www.smashingcamera.com/2017/03/canon-77d-vs-70d-comparison/I don't normally throw out a bunch of links, but in this case there are so many comparing these two models.
To highlight a few things....
70D is a 5 year old model, while 77D is about one year old.
In spite of the similarity of their names, the 70D in its day was a higher specification camera. For example, it has a top shutter speed of 1/8000 and a flash sync of 1/250, and a shutter durability rating of 100,000 actuations. In comparison, the 77D has a top speed of 1/4000, a 1/200 flash sync and is probably rated for around 75,000 clicks (I haven't seen any "official" rating by Canon).
The 70D has a true pentaprism which makes for a slightly bigger and brighter viewfinder than the 77D's, which uses a penta-mirror to keep costs and weight down. 70D also has more sealing for weather resistance.
Regardless what Canon calls it, the 77D is actually a "top of the line "Rebel" series camera. It superseded and replaced the Rebel T6s.
In some ways the 77D might be considered superior to the older 70D. For example, it uses a newer 24MP sensor, versus the 20MP used in the 70D (which is actually still used in the 7D Mark II). 77D also has a two generation newer image processor. The 77D has slightly higher ISO range (but you have to decide what's actually usable for yourself).
And 77D inherited much of the 45-point AF system that was introduced in the 80D. That system is all cross type and has up to 27 AF points that are "f/8 capable" (meaning that it can use more lens/teleconverter combos) and is able to focus in 1 or 2 stop lower light than the 70D. The 70D uses a slightly "dumbed down" version of the 19-point AF system that was introduced in the earlier 7D. That system is also all cross type, but is "f/5.6 limited" and able to focus down to approx. -1EV.
70D was the first Canon model to get Dual Pixel AF (DPAF) in Live View. This uses pairs of pixels embedded in the image sensor itself, to perform autofocus much faster than the contrast detection method used on earlier models. The 77D has this feature, too.
The 70D has Micro Focus Adjustment feature, which allows you to fine tune focus accuracy of specific lenses on the camera. The 77D doesn't have this feature.
70D uses a larger LP-E6 battery that gives over 50% more shots per charge than the smaller LP-E17 that the 77D uses. ALSO, the 70D can optionally be fitted with a Battery Grip (BG-E14, also fits 80D), which doubles the battery capacity AND provides secondary controls convenient for shooting with the camera in portrait orientation. Canon DOES NOT offer a battery grip for the 77D. Vello makes one, but it doesn't have the full set of controls (shutter release only, and that requires and external cable connection).
There are some other differences... and similarities... you may want to consider.