They might be referring to RTL (resistor-transistor logic). A transistor in the circuit can maintain the same output current that was input. (A transistor in fact a diode and a half.) RTL was superseded by TTL (transistor-transistor logic) but, hey, the Apollo computers that put astronauts on the Moon used RTL logic.

You could start with the late Don Lancster's book [1].

I have a little "breadboard helper" that I am wrapping up (that includes a project manual) for creating RTL circuits and others [2]. (I hope to sell a few.)

RTL book [1]: https://archive.org/details/RTL_Resistor-Transistor_Logic_Co...

Prototyping [2]: https://cdn.bsky.app/img/feed_fullsize/plain/did:plc:oxjqlam...