Is it necessary to buy a camera bag?

A camera bag is still necessary. As for the brand, it depends on your choice.

You can buy a shoulder bag or a mirror, which will be more convenient. It will be much easier to buy a shoulder bag for hiking.

If you just use street shooting, you can buy a shoulder bag with one camera and one mirror. It's best to put it in the bag when not in use. The size depends on your figure+the size of the lens.

Or you can buy a stand-alone waist hanging system of Tarantula brand, close to 1000, and you can find photos online. Suitable for hiking without buying a bag. I bought one, which is very good for your reference.