You've learned digit DP. You can now:
Count numbers in huge ranges without brute force.
Handle tight bounds to stay within limits.
Track used digits with bitmasks.
Apply digit DP in binary for bit-based constraints. The observation: build numbers digit by digit, tracking just enough state to enforce your constraints. This turns impossible problems into easy solutions. For training, solve the problems here's problemset. Then move on to Game Theory DP where you'll handle two-player games.