I'm not the best at explaining things but here we go, I also like to use images.
Grab AssetStudio Use this version if you have rigging issues try this it's an auto build of the latest commits Of AssetStudio
1. Exporting Models
Go to the Unity Game's Folder GAMENAME_Data.
File > Load File/Folder select the file/folder with the assets you wish to open.
Use the Scene Hierarchy to export models, select the model you want to export. Or just use the 'Model > All 3D Objects (Split)' option and look through'em with Noesis
Once Selected go to Model > Export Selected 3D Objects. They export in FBX format, so use Noesis to convert to dae if your preferred program can't open it. You may need to select 'FlipUVs' when converting with Noesis.

Or you could just export the mesh from the asset list which doesn't export the rigging.
1.5. Exporting Models with Unity Asset Bundle Extractor(UABE)
If AssetStudio for whatever reason why it does, rips a model in its first frame of animation, grab UABE
First in AssetStudio find the mesh of the model you're trying to rip, right click and select 'Show Original File' then in UABE click File > Open then select the file in the folder AssetStudio opened.
Then find the File Type 'SkinnedMeshRenderer' and select it, then click 'Plugins' and select Export to .dae and OK.

2. Exporting Textures, Sprites, and Sounds
Usually exporting the model also exports the textures, but sometimes it doesn't export the correct textures.
Go to the Asset List, find and select the Texture(Texture2D)/Sprite(Texture2D/Sprite)/Sounds(AudioClip) you wish to export, then go to Export > Selected Assets

3. Compressed/Cached Assets
Some games have their assets compressed as .unity3D / .assetbundles / .bundle / .asset /, or in a 'StreamingAssets' folder, and sometimes without a file extension open the asset up in a hex editor to see if it has the 'UnityFS' or 'UnityWEB' Header then you can continue.
Go to the Cache/Unity3D folder other than the cache, this is different for most games. You can find the cache data in User/AppData/LocalLow/Unity/WebPlayer/Cache. Cache assets files are usually named __Data. Extract Folder won't detect anything without a file extension.
File > Extract Bundle/Folder then select the Unity3D/__data file/folder you wish to decompress.
It should spit out a asset_name.unity3D_unpacked folder with a CAB-NameOfAsset file inside, then just repeat the processes above.

For mobile games it's the same, however the directory and files are a little different. Assets > Bin > Data instead of GAMENAME_DATA, or if it uses an OBB just rename the OBB to .zip and unzip it.
Congrats! Now you (Hopefully) know how to rip from Unity Engine games!