Making infinite terrain

You can do infinite terrain in fact. You should use fractal generation of heighmap in realtime.

Generate heighmap on heighnour terrain and update it when hero moving and you will get infinite

You cannot make infinite terrain. You can make it real long so that it looks infinite but technology does not allow infinite for computers.

Now to answer your question, I have not play subway surfer but it looks like this old thing we had on tv in the 80's with a guy called Hugo and a mine cart.

The easiest way would be to have the guy not moving in forward space, just left and right. Then the world is moving in front of him. Then when you reach a certain point of the level, let's say 1/2 of it because then you would start seeing the edge of it, you instantiate a new portion of level adjacent to the other one. When you reach the end of the old portion, you remove it. And you repeat that "infinitely". On top of that, Subway surfer seems to use a fog to hide the end of the track and the new one created.

The reason why it is better to keep the guy static is because if you go for reaaaaal long, then when you reach 10000, float value loses accuracy (Unity will show a warning). You can still start from -10000 until 10000 which is about 20km walk (semi marathon) but I would recommend the static guy instead.

An alternate way of doing it is to create a long 'Plane' 3D object (Scale: X= 50, Y=1, Z=500; Position: 0, Rotation: 0). Let us call it 'Ground'. Create a green material and apply it to the plane to give it a green landscape look.

Create another plane in the middle (Scale: X=1, Y=1, Z=500; Position: X=0, Y=0.2, Z=0, Rotation: 0). Let us call it 'Path'. Create a dark-grey material and apply it to the path.

Move the Main Camera (Position: X=0, Y=10, Z=-1809; Rotation: X=20, Y=0, Z=0).

Place a capsule 3D object on the path (Position: X=0, Y=1.5, Z=-1800) and make the Main Camera as the child of the capsule object.

Add three random objects on the path: Cube (Position: X=-3, Y=1.5, Z=-800), Sphere (Position: X=-4, Y=1.5, Z=-1700) and Cylinder (Position: X=3, Y=1.5, Z=-1300)

Add a RigidBody component to the capsule (Check 'Use Gravity' and under 'Constraints > Freeze Rotation', check X, Y, Z).

Finally, add the following C# code to the capsule object.

 using UnityEngine;
 using System.Collections;
 
 public class PlayerBehaviour : MonoBehaviour
 {
     // Update is called once per frame
     void FixedUpdate()
     {
         transform.Translate(Vector3.forward * 0.75f);
         if (Input.GetKey(KeyCode.LeftArrow))
             transform.Translate(Vector3.left * 0.25f);
         if (Input.GetKey(KeyCode.RightArrow))
             transform.Translate(Vector3.right * 0.25f);
         if (Input.GetKey(KeyCode.UpArrow))
             transform.Translate(Vector3.up * 0.25f);
         if (transform.position.z > -700f)
         {
             transform.position = new Vector3(transform.position.x, transform.position.y, -2000f);
         }
     }
 }