If ablution is nullified by touching the penis during ghusl, is it obligatory to repeat both ablution and ghusl, or is it sufficient to repeat only ablution before prayer? And is it necessary to perform istinja' before every ablution?

If a person performing ghusl touches their private parts during the ghusl after having completed the ritual washing of the wudu organs, their ghusl is not invalidated. They must perform wudu for prayer, but they do not need to repeat the ghusl. Similarly, they are not required to perform istinja for wudu unless there is an impurity that needs to be removed.