Zone 2 training is a good use of time for amateur endurance athletes.

https://www.youtube.com/watch?v=QQnc6-z7AO8

Post exercise muscle soreness is caused by lactic acid buildup.

https://my.clevelandclinic.org/health/body/24521-lactic-acid

Icing is an effective treatment for minor musculoskeletal injuries.

https://www.yalemedicine.org/news/rice-protocol-for-injuries

Weight training for children will stunt their growth.

https://peterattiamd.com/belindabeck/

Stretching before exercise prevents injuries.

https://mcpress.mayoclinic.org/nutrition-fitness/does-stretc...

I could go on and on but I think you get the point. Almost everything you hear in this field is opinions, extrapolations, and educated guesses rather than anything we could really call a "fact".