If you use the certbot certonly --webroot method, it will make no changes to your apache ( so won’t change things to the apache default page or anything)
You will need to configure your apache for https afterwards - https://mozilla.github.io/server-side-tls/ssl-config-generator/ is a good site for giving you a good config. Let us know what you want a hand with, and preferably paste what you have so we can tell you the modifications needed.