Does building an app or software system count as computer science research?
What counts as research is a matter of opinion, and ultimately, up to the community of researchers who do research in that area.  It will depend on the novelty of the work, what we learn from it, and what has previously been done.  Generally, just building software usually isn't considered research if no new generalizable knowledge is created; but it can be considered research if it is aimed at developing new knowledge (e.g., evaluating a new technique for solving some problem we previously didn't know how to solve).

You are right: just building a useful app isn't normally considered research.  However, this can be tricky to tell from the outside, because developing a new technique to solve some problem can be research, and if you have a new idea that you think might solve some problem, often the way you to evaluate that idea is to implement it and see whether it does indeed lead to the claimed improvements -- which from the outside can look like you're "just" building a useful app.

So, telling research apart from engineering can sometimes be a bit tricky, especially for someone new to research.  Also, understand that this is somewhat subjective.  One person's research can be another person's "mere engineering"; the community doesn't always agree on what ought to be publishable.  Ultimately, this comes down to a judgement call, one that has to be informed by deep knowledge of the area and also by a sense of "taste" that is developed through experience.  We're unlikely to be able to answer whether a specific project of yours counts as research, because we lack that deep domain knowledge of the area, and because the description in the question is too shallow to tell.  If you're new to research, you might also have a hard time judging this.

If you're new to research, this is why you work with an advisor who is experienced in research.  Identifying useful problems to work on that are research-oriented and likely to lead to publishable results is one of the primary roles of your advisor/mentor.

Of course, if you don't know whether you can trust your advisor, and you don't know how to make those judgements yourself (because you are new to research and not an expert in that area), then you have a more basic problem.
Therefore, let me suggest a useful proxy.  Is the professor's work getting published in reputable peer-reviewed forums?  If it is, then at least that research community considers it research, which is good enough.  If it isn't, then you have reason to be wary.  Thus, with this idea, you can look at the publication record from that group over the past 3 years or so and see what their track record is.

You might also find it interesting to read a few of their past papers.  Often, published research papers try to identify what their novel research contribution is (either explicitly in the introduction, or implicitly when they discuss related work and compare to it).