[rich_sasha]

Doesn't really answer you question but IME this is sort of unavoidable unless you're massive and you can afford to have people who just document this kind of stuff as their job.

Reason being, a lot of this stuff happens for no good reason, or by accident, or for reasons that no longer apply. Someone liked the tech so used it - then left. Something looked better in a benchmark, but then the requirements drifted and now it's actually worse but no one has the time to rewrite. Something was inefficient but implemented as a stop gap, then stayed and is now too hard to replace.

So you can't explain the reasons when much of the time there aren't any.

The non-solutions are:

- document the high level principles and stick to them. Maybe you value speed of deployment, or stability, or control over codebase. Individual software choices often make sense in light of such principles.

- keep people around and be patient when explaining what happened

- write wiki pages, without that much effort at being systematic and up to date. Yes, they will drift out of sync, but they will provide breadcrumbs to follow.