How can a person who collected money under the pretext of donations and used a portion of it for personal gain during a time of need due to war repay these funds, and what is the way to repay them to entities that no longer exist?

You erred by deceiving people to unlawfully consume their wealth. What is incumbent upon you is sincere repentance. This means seeking absolution from those whose money you took and explaining to them that you took some of it out of need, or you donate this money to the charitable organization it was intended for, or you return it to the donor, along with seeking Allah's forgiveness and regretting the betrayal of trust. As for other funds whose intended recipients no longer exist, hand them over to those responsible for organizing activities so they can be used for the purpose for which they were donated. Your repentance is not complete unless you seek absolution from injustices. You are not required to disclose what you did if you return the money to its rightful owner.