Differences between 2D,3D and 2.5D in Unity Game Engine

You can create 2D and 3D content separately, use 2D and 3D elements in the same project, or make a 3D game that simulates a 2D view in 2.5D by using the Unity game engine. The main distinction is the type of objects you want to use in your scene and the camera you're using.

Let's see more details of the differences in the following :

Characteristics of 2D video games :

Sprites are flat graphics used in 2D games that lack three-dimensional geometry. They are drawn to the screen as flat images, with no perspective provided by the camera (orthographic camera). Hollow Knight by Team Cherry, Children of Morta by Dead Mage Inc, and Tiny Bubbles by Pine Street Codeworks are a few examples of 2D games created in Unity.

Characteristics of 3D video games :

Three-dimensional geometry is typically used in 3D games, with Materials and Textures rendered on the surface of GameObjects to make them appear as solid environments, characters, and objects that comprise the game world.

3D games typically use perspective to render the Scene, so objects appear larger on screen as they get closer to the camera.

2.5D - Yes, there’s also 2.5D! :

Some 2D video games use 3D geometry for the surroundings and characters, however avert the gameplay to two dimensions, e.g., the digicam may additionally exhibit a side-scrolling view, however the participant solely strikes in two dimensions. For these form of games, the 3D impact has a extra visible as an alternative than practical purpose.

There are additionally video games that simulate 3D geometry and a depth axis, however use an orthographic digital camera alternatively of a viewpoint one. It’s a frequent method that offers the participant the bird’s-eye view of the recreation action, and is frequently referred to as isometric view.

The choice between starting in 2D or 3D mode determines some settings for Unity, similar as whether images are imported as textures or pucks, and if the camera systems is orthographic or perspective. 

Alongside all of the features for 3D development, Unity has a comprehensive point set for 2D games, including a Sprite editor, 2D Drugs, a renderer or Sprite masks, World- erecting tools like Tile map editors for square, hexagonal and isometric penstocks, bone- grounded vitality, and the possibility to fluently produce 2D lights and shaders. You can read further here . 

Now what are the concerned changes made between 2D and 3D ?

Generally it's all about physics components and animation settings , the behaviour of a sprite character (x,y) would have less time in production and a different game play environnement from a 3D character, also components are specialized in 2d like Rigidbody2D and Box collider2D , unity engines has a 2d collection gameObject and 3d collection which are separated from each other and for each collection its own component collection. 

It may seem complicated but believe me once you start playing with all what I said you will have no gaps around.

I will conclude with an advise you may need it: whatever the game dimension you are trying to develop, the  import thing you should handle is to imitate reality ,your video game will go viral due to this secret , gaming industry is being an art life time .