The character じ can also be pronounced as ぢ though in some situations: Loan words (カタカナ), at words' starts (Especially when modified by ゃ ゅ ょ), and before nasals (じん).
This is what happens if you literally add voicing to し and ち, but this is not true of modern, standard Japanese. In standard dialect, either じ or ぢ could be pronounced either way and are not distinguished from each other; this split only happens in the Kagoshima dialect, which is largely not mutually-intelligible with other Japanese dialects.
Orthographically, the main reason ぢ still exists is to represent ち being affected by rendaku, such as in words like 身近 (み ＋ ちか ＝ みぢか, pronounced みじか). There are very few ち words this happens in, so 99% of the time you will encounter this sound written as じ instead.
