There is a bubble-like thing on the foreskin of penis, which has not been itchy or painful for many years. What is this?

Hello, your acne should be folliculitis, don't worry. Folliculitis is an inflammation caused by Staphylococcus aureus infecting hair follicles. Pubic folliculitis is often caused by decreased immunity, poor cleanliness of vulva and secondary bacterial infection (mostly Staphylococcus aureus infection). I suggest you keep your vulva clean and take a bath frequently. Limit alcohol and spicy food. Diet should be rich in vitamins, trace elements and high protein, but also pay attention to pudendal hygiene and take more baths.