What are the difference and the purpose of .dateFormatter() and .dateDecodingStrategy.

dateDecodingStrategy indicates how a decoder should treat dates in the data being decoded using one of several built-in formats, like .iso8601 or .secondsSince1970.

.dateFormatter isn't a thing on a decoder; .dateDecodingStrategy = .formatted(...) allows you to tell a decoder to use a specific format other than one of the built-in ones by supplying a DateFormatter object.

And what actually is the difference between a format and a style?

A style is locale-dependent, so the date will be displayed in a format according to whatever locale is set on the user's device. For instance, if the user's locale is en_US then .dateStyle = .short will be M/dd/yy but if their locale is en_GB it will be dd/MM/yyyy and if the locale is bs it will come out as 19. 9. 2021..

A format is fixed regardless of the user's locale. So yyyy-MM-dd will always show that way whether the locale is en_US or en_GB or nl or mfe_MU or ko_KR or whatever.