Ok. So I was wondering how can Russia develop countermeasures to US MIRVs if it doesn't have detailed knowledge about the way MIRVs manoeuvre. As a Russian this is of concern to me.
First of all if anyone tells you about you being safe under an ABM umbrella then they are lying to you.
The game of measure and counter measure is continuous and ongoing.
Think of a MIRV as a glide bomb rather than a manouvering cruise missile.
When released from the bus they are basically released heading directly at their target with some manouvering capability to fine tune that accuracy and hit the target more precisely.
A MaRV is an actively manouvering reentry vehicle that can perform active manouvers to evade ground and air based threats and head specifically to a point target.
Because it has its own propulsion and guidance it can hit targets much further from the flight path of the carrier rocket and can change its flight path significantly more than a simple MIRV can.
Much more difficult to hit.
Hitting a MIRV is like a TOR hitting a laser guided bomb... a laser guided bomb will perform small flight corrections to ensure a precise hit on a point target but will not manouver all that much.
In comparison a MaRV is actively trying to evade the defences around the target...