How can I download game add-ons from PC onto my Xbox Series X?

Not really...

Xbox Series X games / upgrades / updates has to be installed onto the Series X internal or expansion card SSD,

You can have Xbox One games on your external hard drive that will get an Series X upgrade but the games will need to be transferred to the Series X and then the Series X upgrade will need to be downloaded onto the Series X SSD,

As an example...

I have Marvel Avengers on my external hard drive that will get the Series X upgrade and I will transfer the game over to the Series X SSD ready to download and update the game to the Series X version when available.

Hi, I hope you're doing great out there. I am an Independent advisor and I am here to help you in regard to your concern.

I would like to clarify, are you seeing the DLCs on your manage games and add-ons page? To check please follow the steps below:

Go to “My games and apps” > Installed games > Highlight the game > Go to manage games and add-ons > Click the game icon.

Make sure that you’re signed in under the same Microsoft account where you purchased the DLCs.

Make 1 of the Xboxes the "Home Xbox". When done make sure privacy and sharing preferences are set on the "Home Xbox" to share. After that all the games on the "Home Xbox" are accessible by any profile added to that "Home Xbox. The other (non "Home Xbox) has to have the game purchased under that 2nd Xboxes main profile or purchased as a gift from any profile and sent to a chosen profile to be downloaded when that profile signs onto their Xbox. I have an Xbox One X in my living room controlled by my 2nd account profile. I have a Series X in my game room "Home Xbox" controlled by my 1st profile. Example; I log on the Series X with that main profile and buy/download Division 2 with all expansions and passes. I then log on the One X with my 2nd profile (main for that One X) and buy/download Division 2 with out the expansion and passes. So now a family member adds their profile to the One X, if they own the Division 2 they can play it and as much of the content that I have purchased. To play the full version on the One X someone has to buy it for that console, However anyone else that logs onto the One X with their profile that doesn't own any of Division 2 content can't play it at all. They can launch it but it will direct them to the store. Now adding Ultimate Game Pass to the One X allows profiles to download and play any games available through game pass. Remember my Series X will allow any profile added and signed onto can play any games and use the Ultimate Game Pass to download and play from my Series X. One X has limitations for profiles due to that profile owners games owned and which games are in the One X. Confusing ........ ?

How are you able to download it onto your PC as well? I'm subbed to Game Pass Ultimate, but I don't have an option to download it onto my PC. Only my two Xbox Ones

So so long as I long into the Xbox Series S/X with my account, the one I use to buy the game, I download the game onto the console? I’m just making sure because the Xbox series S belongs to somebody else in my house hold, but I’m still able to use it even though it’s not mine. I was just trying to see if I logged into the series S with my account I could just download the game on there and then play it from there.