Please search the forum - your questions has been asked before - in several variations - so you will be able to find the answers.
- system settings
- firefox is firefox
- nothing is done automagicly - you are in charge and you are responsible
- linuxatemyram.com - probably cache which needs to be flushed or processes to be closed baloo indexer is shutting down
- yes
A few words of advise
As a first time Linux user should get to know the system - it’s limitations - how it is maintained - especially when using KDE Plasma or Gnome - these environment are very picky and can be easily broken when you don’t know exactly what you are doing.
Don’t follow random video tutorials found on youtube or follow random advise or tutorials found on the internet - these are usually only working for the authors system and may not be applicable for yours.