Culture is a garden, and your people are gardeners. Every decision, every meeting, every email, every priority, every exception--each is an act of gardening. Every line of code written, or removed. Every test. Every tool. Every deployment. Water this, yank that, prune here, fertilize there. Some idea gets planted or trod down. Some relationship is nurtured or ignored.
You can't create culture, or design it, or decree it. Even as a leader or manager, you can only tend your plot. Occasionally, you can add or remove a gardener, and this can have a big impact. But in the end the culture you grow depends fully and only on what gets planted and nurtured on a daily basis, and what gets stepped on or neglected.