How can I curl -IL all the website pages starting with the root pagenot just 1 page at the time?

That's WordPress what you using there. I'm sure there are better ways to warm up the cache: https://de.wordpress.org/plugins/search/cache+warm/

Create a file like warmup.sh with the following code

#!/bin/bash

# URL of main Sitemap
sitemap_url="https://www.vgopromo.com/wp-sitemap.xml"

# Extract all Sitemap URLs
sitemap_urls=$(curl -s "$sitemap_url" | grep -oP '(?<=<loc>)[^<]+')

# Loop over and retrieve the individual URLs
for sitemap in $sitemap_urls; do
    urls=$(curl -s "$sitemap" | grep -oP '(?<=<loc>)[^<]+')
    for url in $urls; do
        curl -IL "$url"
    done
done

This will do what you requested.

You can also a Cronjob that run this file.

# Example: At minute 15 past every hour.
15 */1 * * * /bin/bash /root/warmup.sh

EDIT

This modified code adds the option to define also subdomains.

#!/bin/bash

# Array of Subdomains, just extend in same princip
subdomains=("www" "subdomain_2")

# Loop over Subdomains and retrieve URLs
for subdomain in "${subdomains[@]}"; do
    sitemap_url="https://$subdomain.vgopromo.com/wp-sitemap.xml"
    sitemap_urls=$(curl -s "$sitemap_url" | grep -oP '(?<=<loc>)[^<]+')
    for sitemap in $sitemap_urls; do
        urls=$(curl -s "$sitemap" | grep -oP '(?<=<loc>)[^<]+')
        for url in $urls; do
            curl -IL "$url"
        done
    done
done

Ill personally suggest this way. Makes it compatible with multiple projects

#!/bin/bash

# Array of Domains, just extend in same princip
domains=("https://www.vgopromo.com/wp-sitemap.xml" "https://example.vgopromo.com/wp-sitemap.xml")

# Loop over Domains and retrieve URLs
for domain in "${domains[@]}"; do
    sitemap_url="$domain"
    sitemap_urls=$(curl -s "$sitemap_url" | grep -oP '(?<=<loc>)[^<]+')
    for sitemap in $sitemap_urls; do
        urls=$(curl -s "$sitemap" | grep -oP '(?<=<loc>)[^<]+')
        for url in $urls; do
            curl -IL "$url"
        done
    done
done