Is repentance and giving in charity the equivalent of what was taken from the apartment sufficient, or is it obligatory to return the money to the owner, and is it obligatory to inform him of the reason for the return?

You are not permitted to take anything from the apartment's contents without the owner's knowledge, due to the Almighty's saying: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent." And due to the Prophet's (peace be upon him) saying: "The wealth of a Muslim is not lawful except with his willing consent."

You must seek the apartment owner's forgiveness. If he forgives his right, then you must return what you took to him. If it is damaged or lost, then return its value. Giving it away as charity on his behalf is not sufficient as long as returning it to him is possible.