From Business Intelligence Studio:
File->New Project->Integration Services Project
Now in solution explorer there is a SSIS Packages folder, right click it and select “Add Existing Package”, and there will be a drop down that can be changed to File System, and the very bottom box allows you to browse to the file. Note that this will copy the file from where ever it is into the project’s directory structure.
If you need to open a *.dtsx file in SSDT, then see:
How do I open a DTSX file in Visual Studio 2019?