Powerspikes

It's usually determined by either item spikes, level 6 or scaling.

Power spikes can be a number of things, For example Yasuo's power spike is after he completes his first crit Item. once he finishes he now has 60% crit chance. That's a huge powerspike and that is generally where he starts snowballing. A powerspike can also be certain levels, again yasuo's powerspike is level 6, same with zed and riven. high dmg ulties. Zed also has a power spike at level 3, because he can do his full W+E+Q Combo and proc thunderlords easily on you.

It depends entirely on the champion. Maybe it's from getting their ult, maybe one gets a biggerspike from his ult than others, maybe an important item, you just got look into each individual champion to determine this.

Some obvious ones for big spikes at lvl 6 would be Jax, Gnar, Kassadin, and Anivia since they get a consistent powerboost from it throughout the game, due to super short cooldowns or passives.

gets a small power spike when he gets his first back. He does not really spike so much as he keeps going, while leaving everyone behind.