Can someone ELI5 what Firefox container tabs do and if they offer actual privacy vs theater privacy?

I have a general idea of what they do, but have no clue whether they are successful or work similar to the 'do not track' checkbox. If I open a container link for a information-voracious site, is that site's ability to collect data actually limited to what happens in that tab and that tab only? If I close the tab and reopen another 'contained' tab following a different link, do the previous and new tab share information?

Or do they …

checkbox clue collect container data firefox general information link offer privacy theater track work