1GB Limit Explained

Just to expand and clarify on Q1.

When you click "close and apply" the data is stored in RAM. When you click SAVE to save your pbix file the data from RAM is written into the .pbix file (and conversely loading a PBIX reads the all data from disk and loads it into RAM). Technically you will also see some temp files used when you click "close and apply", but those are never permanently stored anywhere.

Note that the size of the data stored inside the pbix file is likely to be slightly smaller than the memory footprint as there are some objects like dictionaries that are stored uncompressed in memory, but have additional compression applied when writing them to disk. And your pbix includes the metadata for your report visuals (the blog Frank linked to breaks down the structure of a pbix file).