Bungie are looking at one busy October this year, what with Destiny 2 getting the Shadowkeep expansion, Vex-flavoured seasonal content and a free-to-play version for Steam, the last of which comes with the added convenience of cross-save.
One has to give credit to Bungie - ever since they split up Activision, Destiny 2 , the studio have been firing on all cylinders, and the recent 13-day of all three barely registers compared to what players are getting on 01 October 2019.
Bungie have a section on Destiny 2's website detailing Destiny 2: New Light's features, with a special focus on cross-save, which means you can play the same game on multiple platforms, including the upcoming console-less console - Google Stadia.
Of course, this feature will be available in all Destiny 2 versions, not just New Light, but the latter will take some Battle.net-Steam porting .
"Cross Save enables you to choose one Destiny player account (your Active Account) that you can access from wherever you choose to play. Although this is not a permanent action, we did build this new feature as something that players will activate just once to support a hobby that spans more than one platform where Destiny can be played", the dev wrote in their FAQ.
Note, however, that cross-save does not mean cross-play, so Destiny 2: New Light players won't be joined in their adventures by platforms other than their own. Other than that though, you've got one game that can be accessed and continued from anywhere.
Cross-save will let you link all your Destiny 2 accounts and designate one as your Active Account, which will be considered the main one.
Bungie built Destiny 2's cross-save feature as a one-time decision thing, but it can be disabled if you choose to do so. You should know that disabling it means you're facing a 90-day wait until you can re-enable it again.
As for the purchases you made on other platforms, Bungie explained that "the things you have purchased from your PC or Console stores will remain on the platforms where you bought them. These include games or balances of Silver."