Does PNG to JPG conversion lose quality?

Yes. JPG uses lossy compression. At 90%+ quality, the loss is imperceptible for most images. The tradeoff is dramatically smaller file sizes.

What happens to transparent areas?

They are filled with white (or another background color you choose). JPG does not support transparency.

Yes. The batch converter processes multiple files simultaneously, applying the same quality settings to all.

85% for web images, 92% for high-quality general use, 95% for print. Below 70% is only for thumbnails where size matters most.