Mrz,
They learn the single sounds which all the spellings (or graphemes if u prefer) of their language represent, be they single letters or combinations of them. This enables them to sound out any word accurately, regardless of whether they have seen them or not, or whether they know their meaning.
No other European language has phonic inconsistencies like 'great, treat, threat' or identically looking whole words with different pronunciations, like 'read, tear, minute'.
That's why children of average ability take around 3 years to become moderately fluent readers of English, while in other European languages they do so in 3 - 12 months, depending on their spelling systems.
Finnish spelling is one of the world's best and nearly all Finnish children can read fluently by the end of their first term at school, and more fluently than many English children can after 3 years.
English speaking children have to keep learning the pronunciation of more words even after they have become moderately fluent, e.g. marine, echo, cello, chord.....
In English, both reading and writing need far more teaching and practice than in other languages. Hence also the disagreements about how best to do it.
And parental help and children's innate abilities make a much bigger difference to progress. It’s a bit like different runners having to cover the same distance. When the learning route is extremely short and easy, as in Finnish, all participants complete it in much the same time, with only minor variations. For an exceptionally long and difficult course, as in English, their native talents and disabilities have a much bigger impact on the time they need for completing it.