What you’re seeing is banding/posterization, usually caused by lifting very dark areas too much, especially from a JPEG. In dark tones there are only limited tone steps available, so when you brighten them heavily, the gaps become visible as lines or blocks. Extra compression from services like Google Photos can make it worse.

To reduce it while shooting:

• Expose more appropriately so you don’t need a huge exposure boost later.
• Shoot RAW instead of JPEG; RAW has more bit depth and tolerates shadow recovery much better.
• For difficult scenes, consider taking separate exposures for sky and foreground and blending them.

To reduce it in post:

• Edit the RAW file if you have it.
• Avoid brightening the whole image equally; mask the sky/dark background and leave it darker if appropriate.
• If you only have a JPEG, a little noise/dithering can sometimes hide the banding by breaking up the visible lines.

In short: the main fix is better exposure plus RAW processing, and avoiding heavy shadow lifting on compressed JPEGs.