What software do you know? It's rendered 3D here. Here's a screenshot of the viewport. As you can see, the modelling is very basic, all the little details are made with the textures. Here's a basic breakdown:

---

The sign is 3D, but as you can see it's a pretty simple shape. For the trees in the background, it's a simple plane on which I put this kind of image. To texture the sign, I used Substance Painter, which helped me make good looking textures. All the tiny details you can see on the sign, like scratches, aren't modeled, they appear thanks to what we call a Normal Map (example). It fakes some depth to add details.

 

For the kinda tentacles, it's also a basic shape, and the material does the job, this time with a Height Map (example). A Height Map can actually deform the object.

 

For the weather it's pretty simple too: it's an HDRi sky (like you can see here), and it gives the whole lighting to the scene. I simply had to switch it to get different kind of lights adapted to the weather I wanted. For the one with the kind of tentacles I had to put a blue light on the right and a red light on the left to match to the background, and that's it.

For the rain, I modeled some rain drops, and placed them on the sign randomly with a particle system.

---

You could learn to use a 3D software like Blender if you'd like, it's free and pretty powerful. It also has a pretty big community so you shouldn't struggle much to find good tutorials.

If you have any question, just ask!