Infringement of a patent is the unauthorized use of patented technology. The criterion is whether the technology covered by the other party's patent claims is used.
Note that it is what is in the claims, what is not required by the claims is not infringement.
Your case here, it depends on the scope of protection of patent a and the scope of protection of patent b. B can apply for more features than a as the scope of protection of b. But if the entirety of b uses technology within the scope of protection of a, then it infringes on a.
This is a situation where there can be mutual infringement (e.g. if a wants to use a new feature, it might infringe on b). That's why many large companies have patent cross-licensing agreements with each other.