We'll always be inclined to try to solve problems with technology answers. But I reckon this is a human question with a human answer.
In my experience, the absolute most effective knowledge transfer is pair programming. Point at things. Talk about the design. Explore together. A shared experience is a deeply helpful learning and memory tool!
(Plus this is multiplicative; i.e. knowledge spreads as person A pairs with B for a day, then B with C while A pairs with D, and so on.)
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