I'm still learning, and this is the first time I have to write backend. There is a restriction, I have to use Thorntail. I don't know much about it.
Thorntail Project Generator has a list of available dependencies but the tutorials I've seen uses Maven with different dependencies which are not listed on Thorntail. Can I use those, will it be able build with the command 'mvn thorntail:run, or I have to find those dependencies' Thorntail alternatives?
You can use arbitrary (almost arbitrary) dependencies with Thorntail.
By selecting the io.thorntail dependencies, you assemble the "just enough" application server your application is running on. If you want to use some Java EE technology, such as CDI or JAX-RS, or some MicroProfile technology, such as MP Config, you should use the corresponding Thorntail fraction. That obviously can't cover all the needs of your application, and so you can have arbitrary dependencies on top (such as Guava etc.).