What is the ruling on the sticky fluid that exits after urination and continues for a period, and how does one purify oneself from it, and is it obligatory to change one's underwear because of it?

The discharge mentioned is Wady, which is impure (najis) and invalidates ablution (wudu). It is necessary to perform istibra' (ensuring complete discharge of urine) after it and to wash whatever it contaminates on one's clothes, because the purity of the body and clothing are conditions for the validity of prayer. One should not hold back urine.