I made a diagram to help you understand how the different timelines/columns work:

https://techn.ical.ist/assets/help.svg https://techn.ical.ist/media/zhoMEHOzhT2r6zlz034

@vhf A step is missing:
...
< Does someone of my instance follow Foo ?>
-false->
< Did someone I follow faved or answered to Foo's toot? >
-true-> (Show in FTL)
-false-> (didn't reach instance)