For registering/authenticating to service, SMS mostly. Same deal in Russia in my experience, basically every website/service signup asks for your mobile number and just texts verification codes.
Computers used to be expensive and people had less money back then, so most of the country essentially just directly upgraded to smartphones. Many don't and never used to own a PC outside of work.
But it also just so happens that in both of those countries, you must have your identity attached to any SIM you purchase. So, anything that makes you register with your phone number will indirectly link your real identity to that registration. It must be very convenient for their governments!