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The hiragana equivalent for ー is another vowel letter (あえいおう) to lengthen the vowel. (ー is nonstandard for hiragana, but still perfectly fine.)
Originally there were some slight differences in pronunciation but over time they grew to be completely interchangeable.
With the writing reform pretty much all instances of づ have been replaced with ず (the same with じ replacing ぢ) and these kana are now only really found in a few specific instances of rendaku (when a consonant sound becomes voiced later in a word).
So it is still used when the sound is repeated
And in compounds
気付く・きづく (Where 付く would normally be つく but becomes voiced づく)
馬鹿力・ばかぢから (Where 力・ちから becomes voiced to ぢから)
But in all other instances of "zu" and "ji" it will be ず and じ