What is the main difference between Ftimeline and UTimelineComponent and when should i use one or another?
FTimeline is a structure type containing the data, it just contains data passively (aleast i think i never used that type) like FVector contains coordinates, it also provides functions allowing you to read the data and manipulate it. This type alone does nothing, you need to write code that does something with it.
UTimelineComponent is actor component which have FTimeline and do something with it inside the actor.
Simply once can be used anywhere in code and 2nd is attached to actor, effectively to the world instance too. Whatever you want to use depends on what you want to make and possibly in some cases you could use either of those