
Stardew Valley: cross-buy and cross-save
There is no built-in cross-save. You can copy saves by hand between PC, Mac and mobile, but not from consoles.
Does Stardew Valley have cross-buy?
Unconfirmed
No source states that buying on one store gives access on another.
Does Stardew Valley have cross-save?
Partly
You can copy saves by hand between PC, Mac and mobile. Steam and GOG copies on one PC share a save folder. The wiki says Switch saves need custom firmware to reach, and staff say console saves cannot move to other platforms.
Source
On PC, they're shared between different copies of the game
Stardew Valley Wiki, Multiplayer
crossplatform multiplayer (e.g., PS4 ↔ PC) isn't supported
Official Stardew Valley forum, staff member Odin, 8 December 2020
you can't move console saves to or from other platforms.
Limits of this entry
The wiki is community-run, not the developer's site, and the staff forum reply dates from 2020. I found no developer statement on whether a purchase on one store covers another.