There are many guides to migrating sites or blogs to WordPress.
Some of these are more turnkey than others. WPBeginner offers free migration services under some conditions, others have instructions on how to export data from the source system and reimport to a WordPress site.
Many of the transfer options involve exporting content from one platform, and importing it to WordPress, including the Blogger, Medium and Squarespace steps. The user will need a computer on which to download the content from one site and upload to another.
Transfering additional information to WordPress beyond content pages may involve much more work or not be possible
Moving from an e-commerce service like Shopify or Gumroad can move content and listings, but because a WordPress user must figure out their own e-commerce plugin for sales, moving customer data and past sales data will be much harder.
Content may be copied from Tumblr, but the social graph would not be.
The source code (markdown) for this article can be found on GitHub.