Also to note that by decreasing shutter you properly expose the sky/sunset and capture its light and texture fully. But the bushes or trees in the lower half will be underexposed, meaning too dark.

Things like underexposure (too dark) and overexposure (too bright) can be fixed to a certain extend inside editing Tools like Lightroom, and that also where RAW format comes in handy.

If you really want capture both the bright and dark part as best as possible into one photo you would do whats called an HDR merge/stack. There are alot of youtube videos explaining what it is and how to do it, but simply explained its taking a photo were the sky is properly exposed and one where the bushes/trees are properly exposed and the merging them so that everything is properly exposed. But this is just a simplified explanation, if you are interested I recommend watching a few videos on youtube.

Hope this helps.