Yes, you may use the digital DLC on your physical copy of Civilization VI for the Switch. [source]
Even if you find a physical version of the DLC at a store, it will just be a digital code that you redeem in the eShop, which will then download and install the DLC on your Switch. This will be exactly the same as downloading the DLC directly from the eShop, unless you find the physical code being cheaper.
Remember that the digital DLC will be tied to your account, so you can re-download it whenever you'd like. If you later decide to get a digital version of Civilization VI for the Switch, you can still use the downloaded DLC on it :)
The expansions "Gathering Storm" and "Rise & Fall" are bundled as one under the title "Civilization VI Expansion Bundle", and come at 3.5GB worth of space. They cannot be purchased individually.
You can easily check this yourself by following these steps:
- Go to the eShop
- Search for Civilization VI (or any other game you're interested in)
- Click on the game to enter its page
- Scroll down to see if the game has Downloadable Content (DLC) / Bundle Contents
- If it has, click on the desired DLC to enter its page
- Scroll down to Details and check for file size under Required Space
I don't know if Civilization VI DLC will be released physically or not, but many good-selling games tend to release a physical definitive edition of their games with all DLC bundled in.
This physical edition may still be partially or fully digital as some physical Switch games require you to download parts of the game or the full game digitally anyway, because Switch cartridges tend to be expensive.
BOTTOMLINE: Yes, you can use digitally downloaded DLC for your physical copies of Switch games. Beware of some pitfalls, though, which you can read a bit more on here.