Dying Light

Guys, I know about the game for a long time, but I started playing actively about 2 weeks ago, the question is: How the level of opponents is determined in the game, for example:
I play on difficulty normally, with melee weapons for 1200 damage, ordinary zombies flew from a poke (into the body area), after pumping survival to level 25, sometimes even with 3 pokes they do not die with the same weapon and they began to kick very often, as at the difficulty level it is difficult and higher, I have never heard that survivability affects zombies.
So the level of zombies increases only with the 25th level of survival or, for example, at the 16th level? And whether the level of zombies will increase if the Legend is also pumped.
The same with weapons, which affects the characteristics of weapons: found in chests and vans, or purchased from a merchant or exchanged for receipts.
Thanks in advance, while a newbie in the game has not fully understood this point.