Learn Dutch: De vervallen steiger wordt opnieuw geplaatst. - The dilapidated scaffolding is placed again.

While the prefix her- means "re-" (as in herbouwen for rebuild), you cannot attach it to just any verb. Herplaatsen does exist, but it usually means to reassign someone to a new job or to rehome a pet. For physically placing an object back or building it again, Dutch often prefers opnieuw (anew/again) plus the verb. It sounds more natural in this context than forcing a her- prefix.

Vervallen means "dilapidated". How does the prefix ver- change the base verb vallen (to fall) here?

The prefix ver- often indicates a change of state, frequently a negative one involving deterioration or loss. While vallen simply means "to fall", vervallen means "to fall into ruin", "to decay", or "to expire". In this sentence, its past participle is used as an adjective to describe the ruined state of the scaffolding.

The English translation says "is placed", so why does the Dutch sentence use wordt instead of is?

This is the Dutch dynamic passive voice. Wordt geplaatst focuses on the action taking place (it is being placed right now). If the sentence used is geplaatst, it would be the static passive, focusing on the result of a completed action (it has been placed).

Does the word steiger only refer to construction scaffolding?