On the topic of muscles and protein:
https://nutritionfacts.org/video/plant-based-bodybuilding
https://nutritionfacts.org/video/protein-intake-and-igf-1-pr...
https://nutritionfacts.org/video/anabolic-steroids-in-meat
On the topic of B12. Many omnis are also B12 deficient, 1 in 5 iirc. In vegans all are eventually B12 deficient under the modern hygiene practices and without supplementation. All B12 originates from microbes that live abundantly in the soil. We are very clean with our food and water, so our plant foods and water are mostly devoid of B12. Pasture fed animals get it "naturally", their factory farmed relatives usually get a B12 supplement. I recommend everyone to get tested for B12 (and D) deficiency as it is very common, and supplement accordingly. Vegans with modern hygiene standards for sure need to supplement (or eat B12-fortified foods).
> B12 as deficiencies in it is a silent killer
Yes. So get your blood tested, and supplement accordingly. This is not something unique to vegans (though a lot more common). For good supplements consult: https://www.toxinless.com