Canon EOS REBEL T8i Advanced User's Manual page 185

Magnified View
To check the focus when the AF method is other than [
approx. 5× or 10× by pressing the <
Magnification is centered on the AF point for [Spot AF] or [1-point AF] and on the Zone
AF frame for [Zone AF].
Autofocusing is performed with magnified display if you press the shutter button halfway
when set to [Spot AF], and [1-point AF]. When set to AF methods other than [Spot AF]
and [1-point AF], autofocusing is performed after restoring normal display.
With Servo AF, if you press the shutter button halfway in the magnified view, the camera
will return to the normal view for focusing.
Caution
If focusing is difficult in the magnified view, return to the normal view and perform
AF.
If you perform AF in the normal view and then use the magnified view, accurate
focus may not be achieved.
AF speed differs between the normal view and magnified view.
Continuous AF and Movie Servo AF are not available when display is magnified.
With the magnified view, achieving focus becomes more difficult due to camera
shake. Using a tripod is recommended.
+Tracking], magnify display by
> button (or tapping
185
).