How are Russians able to defend such a huge territory? Also how did they do it in the past?

 Russia’s borders span over 60,000 kilometers and are monitored by the Border Service of the Federal Security Service, staffed by 170,000 personnel. Even with full deployment, posts would be spaced a kilometer apart, assuming three-person shifts. The extreme climate and challenging terrain further underscore the logistical difficulty of securing such vast borders year-round with limited resources.

But, the Border Service is backed by the 400,000 strong Russian Ground Forces:

The 430,000 strong Russian Aero-Space Forces:

The 150,000 strong Russian Naval Forces:

(Image source: Mil.ru)

The 60,000 strong Russian Airborne Troops:

And the 60,000 strong Strategic Rocket Forces controlling nearly 7,500 nuclear warheads:

Russia’s vastness is also one of the reasons why Russia could defend itself from intruders in the past . To control Russia, the intruders had to control Moscow. And one needed a humongous supply line to reach Moscow.