"...so you don't grow and actually get stronger due to continually breaking down muscle without it adapting by getting bigger..."
That shoulda read "...and actually get smaller..." lol.
Anyway...
Macrophages are a type of phagocyte. I didn't go through the entire process, I skipped over some of the less important/boring parts like ubiquitin binding to broken off muscle fiber causes histocytes to be stimulated and then neutrophils are released and are complemented by monocytes which turn into macrophages. Then the macrophages release some growth factors (IGF-2, FGF, prostaglandins, test, hGH) which starts the anabolism of the tissue. The activity of macrophaes also stimulants Type IV pain receptors in muscle (which sense dull pain) which is believed to be the primary cause of DOMS. Then satellite cells differentiate and proliferate and eventually fuse with muscle cells...and this means nuclei are formed and now the protein synthesis required to adapt to the workout are there. The mRNA comes in because nuclei have the mRNA which is then sent out to ribosomes. The mRNA "instructs" the ribosome's to synthesize new proteins......and you grow.