(As a preface I did do all 30 steps, some aren't obvious so please reach out for clarification)
The standard resolution for images made for the web is 72 dpi
The standard resolution for images made for print is 300 dpi
The two files you should always save are a working file (i.e. a photoshop file - psd file) because you can go back and keep working from where you left off. The second file you should always save is an exported image that's been flattened (jpeg or png) which is used for social media posts. You cannot go back and edit or work on exported images.
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