The best doc to start with is the guide for triggers, and windows (and following the links from there). In particular, it says that, even though the default trigger fires every time late data arrives, in default configuration it still effectively only triggers once, discarding the late data:
if you are using both the default windowing configuration and the
default trigger, the default trigger emits exactly once, and late data
is discarded. This is because the default windowing configuration has
an allowed lateness value of 0. See the Handling Late Data section for
information about modifying this behavior.
Details
Windowing concept in Beam in general encompasses few things, including assigning windows, handling triggers, handling late data and few other things. However these things are assigned and handled separately. It gets confusing quickly from here.
How the elements are assigned to a window is handled by a WindowFn, see here. For example FixedWindows: link. It is basically the only thing that happens there (almost). Assigning a window is a special case of grouping the elements based on the event timestamps (kinda). You can think of the logic being similar to manually assigning custom keys to elements based on the timestamps, and then applying GroupByKey.
Triggering is a related but separate concept. Triggers are (roughly) just predicates to indicate when the runner is allowed to emit the data accumulated in the window so far (source). I think this is the closest thing to the original design doc for triggers: https://s.apache.org/beam-triggers
Lateness is another related part of the configuration which is also somewhat separate (link). Even though a trigger might allow the runner to emit all the late data forever, the pipeline can be set to not allow any late data (which is the default behavior), or only allow late data for some limited time. This leads to the default trigger behavior described above. Yes, this is confusing. Avoid using any complex triggering and lateness if you can, it likely won't work as you expect it to.
So the window classes only handle the grouping logic, i.e. what kind of elements have the same grouping key. These classes don't care about when you will want to emit the accumulated results. This depends on your business logic, e.g. you might want to handle newly arrived elements, or you might want to discard them, it's not part of the window. This means there's no special triggers for FixedWindows or other windows, you can use any trigger with any window (even if logically some specific trigger doesn't make sense in context of some window).
Default trigger is just that, something that is just set by default. You should assign your own trigger if it doesn't suit your needs. And it likely won't, except for some basic use cases.
Update
An example of how to use FixedWindows with triggers.
