Hello everyone!
Im wondering, can i install apps like Signal, VPNs apps, and how many Linux commands do i really need to know to be easily work with QubesOS?
You’re always able to install software/apps as long as you know the package/app names with apt-get install command (in Debian-like-templates) or dnf install command (in fedora-like-templates).
Speaking about “how many commands you need to know” it more depends on “if you worked with Linux-Distros” in the past.
If not: it’s hard to understand QubesOS in the first weeks and I don’t would suggest to start with this OS…
If yes: it’s more easy, cause if you read the Qubes docs in the first weeks of working.
Then, you don’t need to know much of the commands, cause all you try out on the commandline is saved in the bash history and so you always can check back your commands (you’ve inserted in the past) by pressing the arrow up key.
The commands you probably need most are the common ones:
ls -la → showing the content of a directory
pwd → showing the current point of your cursor in directory structure
cd → change the current directory to
cp → copy commandy of files and also directories
mv → move command of files and directories
ren → rename <a file/directory)
nano → command to edit files
All other commands - especially the qubes commands you need for working in dom0 terminal, you’ll learn by reading the QubesOS documentation under Documentation | Qubes OS
It will take you around 6 - 20 weeks to learn doing the first steps in working with QubesOS. Take your time! If somethings unclear - just ask! We’re here 
I would recommend flatpak --user installation for Signal.
Yes, have a look here:
Some community guides are available in this forum, for example: