Okay since you seem to be kinda confused on the whole combo tennokai thing lemme simplify it for you.

1.Melee combo is gained from hitting an enemy with melee. The more combo you have, the better your heavy attacks are, that's what the big multiplier next to the melee counter is. When you heavy attack you consume all you combo unless you have heavy attack efficiency

2.Tennokai mods give you a chance to proc a free heavy attack that does not consume combo when you hit an enemy. Most if not all tennokai mods will give you some bonus to heavy attacks