In Live View, your 650D focuses using the imaging sensor, mainly with contrast-detect AF plus limited on-sensor phase-detect help. That system often has to hunt back and forth to find maximum contrast, so it is slower and can struggle in dim light or on low-contrast subjects.

Through the optical viewfinder, the camera uses its dedicated phase-detect AF sensor. That system can judge focus direction and amount much faster, so it usually locks more quickly and reliably.

So the difference is mainly the AF method: Live View uses sensor-based AF, while viewfinder shooting uses the camera’s dedicated AF module.

To improve Live View AF in low light:

• focus on an area with strong contrast or clear lines
• give the camera more light if possible
• use a wider aperture or shorter focal length if that helps brightness
• if AF still fails, switch to manual focus
• consider an external monitor, angle finder, or simply shooting with a wider framing and cropping later when you can’t use the viewfinder

Tapping the screen only chooses the focus area; it does not make low-contrast or dark subjects easier for the AF system to detect.