When exactly do you say "allahu akbar" during salah?

Assalamu alaikum wa rahmatullahi wa barakatuh I am a revert to Islam and have a question concerning the precise timing of when to say "allahu akbar" during prayer. I was told by one party that you say "allahu akbar" **only when you find yourself in an entirely upright position** (i.e., your head points towards heaven; e.g., after you have finished switching from sujud to the intermediate sitting position or directly before you bow down for ruku). A second party told me that you say it **at the beginning of changing from one position to another** (e.g., you say it when you *intend* to rise from sujud and *then* you rise or you do both simultaneously). A third party told me that they are not sure, but that they simply do it when it feels right (sometimes before they change positions, sometimes whilst they change positions, etc.). What is the correct opinion concerning this?