How to create a level generator for an infinite runner

Okay so first create a cube with a width, height and length of the ground object you need and drag it in the project and it is now a prefab now do the same for the obstacles in your game and create a level Manager script like this:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class LevelManager : MonoBehaviour
 {
     //The ground prefab
     public GameObject groundPrefab;
 
     //Reference to all ground objects
     private GameObject[] allGroundObjects;
     // Postion where the new prefab will be instatiated
     private Vector3 instantiatePos;
     // Index for the all ground GameObjects
     private int groundIndex;
 
     //player for Test
     public Transform player;
 
     // Start is called before the first frame update
     void Start()
     {
         //Initialize the level and intantiate the ground Objects
         allGroundObjects = new GameObject[5];
         for (int i = 0; i < 5; i++)
         {
             InitializeLevel();
         }
     }
 
     private void Update()
     {
         player.Translate(Vector3.forward * Time.deltaTime * 5f);
 
         if (player.transform.position.z > instantiatePos.z - 35)
             AddGround();
     }
 
     // Locate the last ground player just crossed to the front most
     void AddGround()
     {
         allGroundObjects[groundIndex].transform.position = instantiatePos;
         instantiatePos += new Vector3(0f, 0f, 10f);
         groundIndex++;
         if (groundIndex >= 5)
             groundIndex = 0;
     }
 
     void InitializeLevel()
     {
         GameObject obj = Instantiate(groundPrefab, instantiatePos, Quaternion.identity);
         allGroundObjects[groundIndex] = obj;
         instantiatePos += new Vector3(0f, 0f, 10f);
         groundIndex++;
         if (groundIndex >= 5)
             groundIndex = 0;
     }
 }

Here I have used a player position as a reference to add the ground you this script will be very performative as you are just changing the position of previous ground Note: I have added a 10 unit in instantiatePos you add the length of the ground in the Z Axis otherwise they might overlap. another thing is add the script to a empty gameObject not the ground itself or the player.

and for the obstacles just do the same instantiate some in the start of game and Later change their position as you need or you can use Unity's Random.Range function

Hope this script gives you the idea I have kept everything basic as you are a beginner.