Jellyfin used to open for me as its own window, separate from Chrome, but now every time is just opens as a Chrome tab and I can't find any way to change it back.
One solution is, and I'm not sure if this is what you are looking for, install it as a Chrome app.
Open Jellyfin in Chrome.
Navigate to the website you want to add as an app.
At the top right, click More (the three dots).
Click More Tools.
Click Create shortcut.
Enter a name for the shortcut, open as window check marked and click Create.
It should then ask you if you want to pin it anywhere, etc. and give you more options
Didn't have that option under More Tools, but I found and install option under 'save and share' and that got it to go back to a separate window, with the pin I had already created, all back to how it used to be, so thanks!
