@DadOnGuard …sorry if this is a bit simple, but you’re asking all the right things.
Those gentler apps like Paired and Lasting mostly keep things private between the two people using them. The questions, answers, and exercises are usually only visible on your own phones, not to the public. It’s still wise to: use strong passwords, turn on two‑factor login, and avoid sharing account details.
For location apps like Life360 or Spynger, both people absolutely should agree first. Installed secretly, they’re not “trust‑building” at all, more like spying. If a couple does use them, it should be after a very clear, honest talk about boundaries and what’s shared.