If they delete your domain then only your domain will be lost not the whole website that you have made
have they authority to delete my domain
No i think they don’t have
You can create a app (or website) which has 000webhostapp subdomain first. Then you can park or point it to 000webhost.
If they delete your domain, you will loose your domain. But, your website will remain here and it will accessible through your 000webhostapp subdomain (like yourwebsite.000webhostapp.com). Also, if you buy (or get) another domain from any other company, you can park or point it here and use it as your website’s domain.
Hope it helped
If you have any other questions, feel free to ask