"Independently usable" does not have to mean "totally independent". The libraries could be related to the same product using the same infrastructure.

That doesn't follow. Why should they not be in the same repository? They may be related but independently usable.

mono repo approach (or something to that direction)

Do people use a mono repo approach with different parts of the repo having different version numbers? The way I thought of it, the whole point of mono repo is not that.

Sometimes, yes. Monorepos use monolithic versioning internally, but they may use semantic versioning when publishing libraries.