Why do people say God is going to keep his promises?

To start, I believe in God. But I have noticed some things other christians say that don’t make sense to me. Why do some Christians say God is gonna keep his promises, or that God has promised…? If it’s not on the Bible that he has said he wants to promise something, where do people keep getting this promises? For example, people say “God promised me a house”, “God promised you’ll heal”, “God will fulfill his promises of X”

I asked my mom about this and she said that it’s because God puts desires on your heart. And here is where it may get a bit controversial (again, I am genuinely curious, I don’t want to offend anybody: If you strongly believe in something that you can’t guarantee (e.g. a house, health, etc…), in your mind you’ve convinced God himself promised that to you, isn’t that just self-delusion? You’re creating “fantasies” and adding the name of God as the one in charge to fulfill them. and if im completely wrong with my argument, what do people mean when they talk about God’s promises -unrelated to the scripture–?