Temporary containers has a plethora of settings for when to open a new container. For example, I have a rule enabled that will open links in a new temporary container when they leave the current one. That's a terrible explanation, so let me give an example to make it clear:

- I have a GitHub container

- github.com and gist.github.com are set to always open in the GitHub container

- Say I am currently browsing github.com in the GitHub container.

- If I click a link to a domain other than [gist.]github.com, instead of navigating my current tab to that url, the url will open in a new tab & new temporary container

This is more powerful than simply persisting cookies from github.com -- I'm keeping GitHub's cookies, but only in the github container. It's almost like first party isolation, but a little weaker (unless you enable the setting where any link to a different domain will open in a new container), and I have the ability to group sites that would break with 1st party isolation by opening them in the same container.