I think what could help would be to describe what Nextcloud is purely in terms of software. For example, Nextcloud is a LAMP software consisting of Linux, web server, database and PHP. Ok there are some variations e.g. with docker, AIO, snap, … Many consider LAMP or PHP to be rather retarded and not scalable. Somewhere I saw a video by Frank Karlitschek which shows why it is still good that large parts of Nextcloud use PHP.
In contrast to e.g. Google or Microsoft, each instance normally has its own web service and therefore name e.g. cloud.server.tld
. There are thousands or millions of Nextclouds and not just one e.g. Microsoft Cloud with thousands or millions of tenants.
On this basis, however, Nextcloud is almost like Wordpress if you are a bit sakastic, except that files are managed instead of websites. There are also many Nextcloud apps that also access files or database content. Nextcloud can do practically everything except make coffee. 
As it is primarily about files, there is also the WebDAV protocol for data transfer. However, as it is a web service, things such as CIFS/SMB or sftp are not available to users. The use of Nextcloud clients makes the transfer of files better or not. In my opinion, protocols such as CIFS/SMB or sftp are not achieved.
As PHP is used, config/config.php
, which describes the system and contains the access data for the database, must first be loaded for each PHP call. Without config.php
and the database, Nextcloud knows nothing. Modern asynchronous JavaScript optimises the exchange between web client and server in Nextcloud.
Nextcloud also offers commands such as occ
, which carry out manipulations on Nextcloud and above all on the database. It might also be helpful if you know a little about the structure of Nextcloud and perhaps also understand the update mechanism a little, even if you should never actually customise it.
I hope I haven’t forgotten anything. Once you know all this, you can start reading the documentation …