Why in the act of cutting the bread in Shabbos With a knife or broke it with my hands, It’s not a melacha of Shabbos?
The reason is because although we don’t grind things on Shabbos because of Mileches Tochain, however this melacha will not apply to bread because the wheat was already ground. There is a rule that once something is already ground, even if it stuck together again it is still permitted to grind it afterwards. This rule is called אין טוחן אחר טוחן- there is no tochain after tochain.
Rema O:CH 321-12.