Why does skin pigmentation occur?

Recently, my freckles and blemishes have become increasingly darker, making my overall skin tone look dull. I'm considering skin pigmentation treatment. I've been using UV protection and whitening products regularly, but the pigmentation doesn't seem to be fading easily, so I'd like to receive professional treatment advice. I've heard that skin pigmentation treatments vary depending on the cause, and I'm curious about what kind of skin pigmentation treatment is best suited for my skin type. I'd also like to know whether skin pigmentation treatment primarily involves lasers or toning, and whether skin pigmentation may temporarily appear darker after treatment. Please also provide information on the number of treatments required to improve skin pigmentation, the recommended frequency, and management methods to prevent recurrence.