Php and mysql database hosting questions.

Php and mysql database hosting questions.
I was using only the previous html css javascript (jQuery).

Not long ago, I learned the use of php and database by self-taught.
Write a site with php (ajax) and learn later (ajax works the same way as fetch, right? Displayed on the screen without refreshing...))) That php used to get data from the database.
I wanted to have access to my site from other devices (other networks).Therefore, the port forwarding http80 was left open to access my bitnami Apache htdoc folder.
It worked successfully and was satisfactory.
However, human greed is endless.
And security issues...
I wondered if my small, old, cracked laptop and home server (router, commonly referred to as Wi-Fi) could withstand requests from numerous devices.

So I found the keyword 'hosting' through search.
Hosting means putting a file on the hosting server (company) and the user communicates with the server, right?
You will send or refer to the data in the database through php. How can I mount php and database on different servers so that they don't come to my area (I mean this small old home Wi-Fi and laptop)? And finally, what I want to create is not a website, but an application using web technology, and I want to post it on the Google Store (in the form that the address bar is invisible to the user).
I want you to tell me free websites or technical keywords about this.