I have recently decided to learn DBMS and SQL starting with the oracle database (express edition/XE). I came across its versions such as 11g and the new 18c/19c and the most recent 21c, and would like to know the differences between these versions in relation to the other applications such as oracle sql developer and oracle apex. I saw on other forums that sql developer and apex were integrated in 11g but later versions from 18c required it to be installed separately, is that true? if so i wish to know if i would really need sql developer to maximise my experience with database or if database alone is enough (i dont need apex since im not into web applications right now).
And also I'd like to know which version i must install 11g or 18c for learning... if there are other better options apart from oracle to learn sql and DBMS, please let me know.
For a beginner, XE is a good choice. Versions you named differ in architecture, RAM and CPU they can use, amount of data you can store into the database, ... Also, there are new database features (unavailable in 11g). From your point of view, 11g might be enough (but no problem if you want to use modern database; 11g is quite old, after all); you can always upgrade.
I just reinstalled my old 11gXE the other day; it installs in a few NEXT clicks and creates a database for you, along with the HR (Human Resources) predefined schema which contains some tables and data so you can start working on it right now.
Database itself isn't enough - you need a tool to access it. Oracle has, from its beginning, offered a command-line tool named SQL*Plus. There are GUI tools available on the market; Oracle's free tool is SQL Developer - I suggest you download and install it, it'll make your education much easier. What do you do with those tools? Write queries, create tables and procedures, ... such things.
Application Express (Apex) is another tool; it lets you create web applications, even with a limited (PL/)SQL knowledge. 11g contains Apex 4.0.2 (which is rather old, just like the database itself). If you'd want to practice Apex, consider signing in on apex.oracle.com. Oracle provides - for free - some space for your data and applications, and it always runs the last Apex version. Or, if you want, you can download & install Apex in your own database. But, as you said, you don't plan to do web development yet, so I suggest you just remember that there are different options for you.