Hi @levanii
By default on Smush free version, we will only compress your thumbnails, only on the Pro version, it will be able to Smush and original size and use the Super Smush for extra optimization.
Those thumbnails are the version of the image that WordPress create to your original image size, you can find more on: https://wpmudev.com/blog/wordpress-image-sizes/
On how much the plugin is going to compress the image depends on what needs to be optimized, but for some images around 1920px removing the metadata, it can decrease to around 200-300kb yes but really depends on how much we need to compress it.
When a user is using Smush Free which won’t include the original image it is a good idea to run save the image for the Web and using sRGB you can use software like GIMP which is free.
Guide on how we should save the image: https://www.wpbeginner.com/wp-tutorials/how-to-fix-image-color-and-saturation-loss-in-wordpress/
About 1920 X 1080, it also depends, not on all page locations we need this size, for example why using a 1920 px if the container is 500px only, for those cases you can use the thumbnail versions that are lighter too.
Let us know if you have any additional questions.
Best Regards
Patrick Freitas