There are several ways to get from Photoshop or Illustrator into After Effects and then various things that can done afterward. As Footage, As Composition - Retain Layer Sizes, As Composition, and Create Shapes from Vector Layer each offer different ways to work with the footage once in AE. Rather than a standard way, which one is best depends on how you plan to work with the source footage. Take some time experimenting with each option to determine which works best with how you'd like to animate, but also read this Adobe Help article: Working with After Effects and other applications
- people in some cases convert psd and ai file into shapes ( creat> shapes from vector layer).
Illustrator source footage allows us to use Layer > Create > Create Shapes from Vector Layer while Photoshop source footage does not. Interestingly, Photoshop Shape Layers turn into After Effects Masks.
- In some case the take three/four layes and pre compose them.
When we import a layered PS file or layered AI file into After Effects as a Composition, After Effects sees the Layers as the source footage items and the PS file or AI file as a folder contaning them. The Layer Stack in the After Effect Comp may look different than it does in Photoshop or Illustrator, including Pre-Comps (a.k.a Nested Comps) because After Effects is a differnt applicaiton that's attempting to match the combied appearance of the Layers.
- Some times they directly use ai and psd files.
The main advantage of bringing a layered PS file or layered AI file in "As Footage" is that it should look just like it looks in Photoshop or Illustrator. I'd prefer if this was called "As Merged" instead of "As Footage" to better reflect what's happening on the AE side. Of course, if we've imported it As Footage and it turns out we need it to be a layered Comp, we can always select it and choose Layer > Create > Convert to Layered Comp. After Effects can reproduce most of what Photoshop can do with layers or has an equivalent, but not everything. For example, After Effects does not support PS Pattern Overlay Layer Styles nor does it support PS Layer Styles that can be applied more than once (like Stroke and Drop Shadow) in Photoshop on the AE side. Similarly, while After Effects supports nested Comps, it cannot translate Illustrator Sub-Layers into them - Sub-Layers just become merged into the coresponding root level Layer.
If you're creating a lot of artwork in Illusrator, it's definately worth checking out sending AI files to After Effects from Adobe XD ("Working with Adobe XD and After Effects" is covered in the "Working with After Effects and other applications" link above) and/or the well liked $45 3rd party add-on called Overlord by Batlte Axe. While there's some built-in support for copy and paste between AE and AI already, Overlord takes it to another level.