They are on F-Droid, which is an alternative Android app repo for open source apps (in fact I think the team builds the apps from source themselves). In addition to their requirements, they audit apps [1] and have been audited themselves [2]. The repo also lists any concerns (eg if the app has tracking) in the description, though the vast majority have no issues. And afaik there has not been a single case of malware (even though similarly sized repos have had them [1]). F-Droid is by far the best place to get FOSS privacy-respecting apps.

However due to the stringent requirements, you can imagine that the selection is smaller. For simple everyday things like a phone dialer, contacts app, gallery app, calendar, etc, Simple Mobile Tools will show up near the top of the list on F-Droid. A lot of alternatives are just old and unmaintained. Simple Mobile Tools gets the job done and has a decent UI, so you can uninstall whatever sketchy bloatware your phone came with and use these instead.

TLDR: if you care about FOSS and privacy, these are often your best option.

[1]: https://www.reddit.com/r/privacy/comments/3cjj2e/how_secure_...

[2]: https://f-droid.org/en/2018/09/04/second-security-audit-resu...