Are elements in cleanResult always either a URL or a number? In that case, you could just check the first character of the string to see if it's a number (basically a non-url). If it's not a URL, then we know it's numbers, and we can do something with the URL, which should the the previous element:

// If it's a URL, we will store it here for future use
let currentURL = ''

cleanResult.forEach((item, index) => {
  // Get the first character of this string
  const first = item[0]

  if (!Number.isInteger(first)) {
    // This is NOT a number, so must be a URL, 
    // let's store it in our variable to use in the next loop
    currentURL = item

  } else {
    // This IS a number, which means we need to do a few things:
    // 1. Split into separate numbers
    // 2. Create a new URL pattern from each number
    // 3. Push to finalResult

    // 1. Split by tab delimiter (?)

    // splits by tab, and returns an array
    const numbers = item.split('\t')

    // 2. Create a new URL pattern from each number
    numbers.forEach((n) {
      // This should now give you the URL + | + the number:
      // ex: https://example.com/firstlink|500
      const newURL = currentURL + '|' + n

      // 3. push to the finalResult array
      finalResult.push(newURL)
    })
  }
})

I haven't tested it, but this is the process that I generally use: break it into smaller tasks and take it one step at a time. I also didn't use regex, just to make it easier. We're assuming that you will receive either a URL or a list of numbers separated by a tab. This means you can afford to keep it a bit simple.

I'm sure there are way more efficient ways to do it and in a lot fewer lines, but if you're just learning JS or programming, there is nothing wrong with being extra verbose so that you can understand early concepts.