There really isn’t a set rule. Typically if I just want to loop through something a set amount of time or I really want to control the timing of something I find it easier to use a Coroutine. Update methods constantly get called multiple times per second, so a Coroutine allows you to replicate that but you can decide how often to update something.
Usually you can replicate the effect within one of the update methods. FixedUpdate is typically used with physics since it updates every .02 seconds or whatever you have your physics time step set to. For this effect you could include everything within the Update method.
