[untog]

It also seems that bolt.co and Instagram's Bolt are much more similar to each other (apps dedicated to person→person communication)

I disagree. A web browser is used for person to person communication too. All three are used for very, very different forms of it, though.

[umberto]

Trademark is about preventing confusion, not owning words. A browser is very different from an app voice/text app.

[untog]

My point was that the two Bolts barely compete with each other. One is for voice calls, the other is for quick picture sharing. I can't imagine many scenarios where you could use them interchangeably.