Path.Combine(string, string)

Description

Concatenates two path strings.

If path1 does not end with a valid separator character DirectorySeparatorChar is appended to path1 prior to the concatenation.

using System.IO;
using UnityEditor;
using UnityEngine;

public class CombinePathsExample : MonoBehaviour
{
    [MenuItem("Example/CombinePaths")]
    static void CombinePaths()
    {
        string path1 = "Assets";
        string path2 = "example.txt";

        Debug.Log(Path.Combine(path1, path2));
        // The result is "Assets/example.txt"
    }
}