Nobody learns to spell all English words correctly by phonics alone, because 4 words out of every 7 (or 3,695 or the 7,000 most used ones which I analysed) contain one or more phonically unpredictable letters (e.g. speak, seek, shriek).
For reading, learning the different sounds which graphemes like o or ea can have (on, only, other...; eat, threat, break ...), can help with decoding unfamiliar words up to a point, because few have more than two or three different sounds. For learning to read English, phonics works quite well, most of the time.
For spelling, it's a matter of memorising words by word which alternative applies to a particular word. It's largely a matter of imprinting on your brain 'what looks right'.
Teachers can organise them into little groups (group, soup, coupon), but when children do their own writing, they simply have to remember which is right. Learning the main patterns (moon, noon, soon, spoon.. ) is not difficult. It's the exceptions which cause all the hard work and heartache.