A few keyboard commands to use multiple buffers in Emacs:
C-x d to go to a dir
use arrows and enter to navigate to where you want to go
C-x C-b to list buffers
use arrow keys to select a buffer
hit return to go to that buffer
hit "d" on the line of each buffer you want to delete
hit "x" to perform actions
C-x o to go to a different window
C-x 1 (number) to make that the only window
C-x 0 (zero) to close current window
C-x 2 to split vertically
In Emacs, a “window” is another emacs file open in the same terminal. It does not mean “window” in the sense of another open application on your system. You can have multiple buffers open, but only one or two showing in emacs windows. What is called a “window” in other contexts (like OSes) are called “frames” in Emacs. Here is a question on the Emacs Stack Exchange with a picture explaining it.
Usually, buffers are files. But there is also a help buffer, a messages buffer, and sometimes other buffers. For example, if you working with some variant of Lisp, you can hook your REPL into Emacs, and there could be a buffer running your REPL.
You can also list the buffers using M-x list-buffers. One difference is that C-x C-b will open the list in the same window if you only have one window showing. M-x list-buffers will open a second window if you only have one showing.