Newbie: How to save animated animals//storys

I am Newbie.
I did only a wandering star as animation in Synfig.

Qustion: how to comose a video?
I wanna to an aquarium.

Can I create a fish and save it in a file.
Can I create a dancing shell and save it in a file.
Can I create a jumping seahorse and save it in a file.

Can I now compose a video out of the 3 animals?
Or must I create an video and paint all 3 animals and animate then together?

In our country we have the custom
to wait every day during Decembre for Xmas.
So we do a movie and every day the movie is sixty seconds more and on Christmas You have 25 minutes movie.
And we do it with Corel and save the pictures as gif and then do it as gif89.

So can You do the same with Synfig
make created and published movie every day longer?