What storages Teams Apps are using

While I don't disagree with you, this is hardly a "Teams App" issue - it's true for ANY SaaS system you use. Certainly a smaller/lesser known vendor, and even true for the larger/bigger names. While the larger ones might have more to lose (reputation-wise) for anything malicious, they're also very prone to breaches, as we've seen time and again. As a result, there's a certain level of trust that we're either assuming or granting to -any- 'cloud' product. This certification, while hardly faultless, at least aims to make the vendor think about (and hopefully implement) certain practices. In addition, it's of course recommended to review the vendor's privacy policy/terms of use/etc.