Q. what is the best resource for exercise training

Exercise is probably the single most important thing you can do for your personal health. Since everyone enjoys different physical activities (biking, football, golf, to name a few), no ONE source is the "BEST" for everyone, because if you don't enjoy the exercise, you won't stick to it. 

Some online resources:

American Council on Exercise: www.acefitness.org

Centers for Disease Control and Prevention: www.cdc.gov/physicalactivity

Exercise is Medicine: www.exerciseismedicine.org

National Center of Physical Activity and Disability: www.ncpad.org

NIH Senior Health: www.nihseniorhealth.gov/exercise/toc.html

President's Council on Physical Fitness and Sports: www.fitness.gov

Specific websites that are free/partially free:

Yoga: Do Yoga with me some free and some fee based yoga routines

Pilates: https://www.verywellfit.com/free-pilates-exercise-videos-online-2704368

Zumba Beginner: https://www.youtube.com/watch?v=9_MxwQw10RI

Barre Studio https://www.youtube.com/user/barre3sadielincoln/

Fitness Blender https://www.youtube.com/channel/UCiP6wD_tYlYLYh3agzbByWQ

At Home HIIT https://www.youtube.com/watch?v=sih4kBaBGjg