How do we distinguish between the actions of the Prophet, may God’s prayers and peace be upon him, that are considered Sunnah and are to be followed, and those that are merely customs and are not to be followed, especially since the Sunnah is a saying, an action, or an approval?

The Sunnah is what has been authentically reported from the Prophet, peace and blessings be upon him, whether it be a saying, an action, or a tacit approval. The default for his sayings, peace and blessings be upon him, is that they are for legislation, because he is conveying from his Lord. He might say something without intending it for legislation, but that requires evidence. As for his actions, they may be for legislation, or they may be from actions of human nature and habit, or they may be ambiguous between the two, or they may be specific to him, peace and blessings be upon him. Legislative actions are identified by being an explanation of a command or a recommendation, or by an explicit statement of emulation, or by encouragement to perform them. As for actions of human nature, they are everything done in accordance with human essence, and no intention of worship is apparent in them. These are permissible, and there is no harm in emulating them unless they contradict what people are accustomed to or become a symbol of the corrupt.