The VEX Knowledge Base has several explanations on VRC concepts/ideas. The V4 classroom curriculum while outdated, has some good tips too. As always the VEX Forum is a great resource where you team can ask questions about VRC.
As for youtube, I would recommend Kepler Electronics (@ KeplerElectronics) as he has some good explanation videos (like on DR4Bs, cascade lifts, ratchets, ect.). There are some other good channels like Taran Mayer’s (@ TaranMayer) (ratchets & differentials) and Connor White’s (programming concepts) too. I would also look at reveal videos (not just from this season) to get good tips on how teams construct their robots (build quality) and to see some of the designs that teams are using this year.
Finally, if any of them have discord, I would suggest having them join the Unnoficial VEX Discord (previously called VTOW) via this link: https://discord.gg/fv44eYyB
I hope that this post is helpful to your future teams. Good luck competing next season!