Given two strings, A and B, how do you count the appearances of characters from B in A?

"I interviewed for Software Engineer, Machine Learning at the L5 level. My process was a recruiter screen, an online coding assessment, one live coding round, a second recruiter debrief call, and then a final virtual onsite with three coding interviews, one behavioral, and one system design round. The coding rounds were mostly classic algorithm questions, while the system design was a recommendation system for ranking locations in a feed for someone visiting a new city. I actually felt better about the process than I expected because the interviewers seemed more lenient in how they graded me than I would have graded myself."