Oracle 11g XE to 18c XE - What is the difference? Is 18c can be used like 11g XE?
Not sure why they force us to download 18c XE?


They're not forcing you to download 18c XE.  They're just making it harder for you to download older versions


  Is it okay to download and use like 11g XE?


Yes.
I suggest you still read the License Agreement for 18c XE.


  What are the differences between them?


Resource Limits have been increased (information can be found in License Agreement section License Rights)


2GB of RAM (up from 1)
12GB of user data (up from 11)
2 CPU Threads (I don't know if this is the same as 2 CPU cores.  11g was limited to 1 CPU core)


New database features


The ability to handle SCD Type 4 data via Flashback Data Archive (11.2.0.4+)
The ability to handle SCD Type 2 data via Temporal Validity (12c+)
MATCH_RECOGNIZE (12c)
other 12c features
other 18c features


"more or less all EE Features" (list from 18c XE Licensing Information)


EE feature Virtual Private Database (VPD)
Enterprise Manager (missing in 11g XE)
Java inside the database (missing in 11g XE)
Partitioning (EE add-on)
Multitenet (EE add-on - XE is limited to 3 PDBs)
Advance Compression (EE add-on)
In-Memory column store (EE add-on)
Advanced Analytics (EE add-on)


  (I can't find any solid evidence why I should use 18C??)



Because 11.2.0.2 (11g XE) is known to be buggy and full of security holes.

anti-upgrade reasons:  Minimum supported Oracle APEX version is 5.1.4