Geometry is edges and faces. Textures are images that you apply to surfaces. Since you are using the free online version you will be limited as far as what you can do. No extensions so you would have to model it manually. I would suggest not worrying about modeling the fins and just make a smooth tube using Follow Me. Then apply a gray color or a metal texture to it. For your hobby application that should get the job done.
Here's a quick example drawn in SketchUp Free.
Screenshot - 10_25_2019 , 7_42_05 AM.png
A path for Follow Me
Add a circle at the end.
Run Follow Me.
Extend the ends with smaller diameter circles and add a metal texture.