At the moment I use C-xC-f to open a new file, but it gets quite tedious.
When you load a file in Emacs with find-file (bound to Ctrl-x Ctrl-f by default), it gets put into a "buffer". As long as you don't kill the buffer, it stays in memory, you don't have to reload it.
You access already loaded files (buffers) with switch-to-buffer (bound to Ctrl-x b). Press the keys, then type the buffer name you want to switch to (by default that's the basename for any loaded file).
You can also press Ctrl-x Ctrl-b to get a list of currently existing buffers. And click any buffer there with the mouse to switch to it (IIRC, rarely use the mouse with Emacs ;-))
You can also split the screen in two (or more) parts to see different files (buffers) with Ctrl-x 2. Initially, this shows the same buffer twice. Switch to another buffer in one of the windows to see another file.
C-x b and C-x C-b are what you want. Also consider trying iswitchb or ido. Since I started using those I wish every program I used could switch tabs / windows / documents as quickly and conveniently as emacs. Often I have 50 files or more open in emacs, you can find the one you want very quickly with iswitchb or ido.
You can easily open multiple files at once (or act on them in other ways).
C-x dto visit a directory (folder), which lists all of its files and subdirectories.
Mark the files you want to open (or to act on in some other way). You can mark files individually or in groups based on various characteristics, including file-name patterns. See the menu-bar menus Mark and Regexp -- it shows you some available marking possibilities, as well as their keyboard shortcuts.
% mmarks files whose names that match a regular expression.
% gmarks files whose contents match a regular expression.
* *marks executable files.
To open all of the marked files at once you need to have loaded standard library
M-x load-library dired-x or put
(require 'dired-x) in your init file.
Then you can use
dired-do-find-marked-files) to visit all of the marked files at once. Use
C-x C-b to see a list of the open buffers, where you can choose among those file buffers.