Assalamualaikum

In acts given by Islam, how can one recognise between that this act is fixed and here you can do something voluntarily. For example in wudu one must wipe his hair, but if someone is taking a shower and there he just washes his head/head and don’t wipe then is his wudu acceptable or not?

Secondly: is order of washing parts in wudu obligatory. Like one is swimming and after intending for wudu he just go in water, is his wudu acceptable.

Please answer principally and application of that principle on both examples. Thanks