That looks to me more like noise, particularly chrominance noise, than posterization or banding. But the two things can look very similar at times, particularly if too much compression was used when converting to JPEG.¹ There are some noticeable compression artifacts near the edges between the red sleeve and the background.

When one underexposes a shot and then increases the brightness later in processing (for a raw file) or post-processing (for a JPEG), one also increases the brightness of the noise in an image. Working with the raw data (instead of with a JPEG) will allow more significant adjustment of things such as brightness and contrast before banding becomes an issue. It also allows more control for doing noise reduction and sharpening (that can also affect the perceived level of noise in a photo).

Why do my images look fine in Lightroom itself but poor quality when I export them? shows the other side of the coin. As indicated in the comments to that question, heavy compression was the culprit there.

¹ The amount of compression used is determined by the "quality" setting when saving a JPEG file. The 100% setting uses very little compression but will result in the largest file size. A 60% setting will use significant amounts of compression to produce a much smaller file size. Sometimes the choices are labeled as 'Extra Fine', 'Fine', Normal', etc. I've never seen a camera with a choice worse than 'Normal' (i.e 'Low' or 'Poor')! It's like fast food joints where the smallest size drink is a "medium" and then there are the "Large", "Extra Large", and even "XXL" choices. Or shrimp, where the smallest size is "Jumbo" and they go up from there.