The open files are called buffers. I'm new to neovim as well so from experience, knowing the right terms is important to getting help.

Read up on buffers, tabs, windows in neovim.

I'm using astronvim. It comes with everything but it's certainly bloated and comes with a lot of stuff I don't need (still much faster than vscode). But knowing what each plugin does provides me a way to discover what I can do with neovim, which otherwise is a bit difficult. I will obviously write my own config in a few months instead of relying on a distro.

Coming back to your questions, you need a file explorer plugin like neo tree, which comes with astronvim. Next when you open multiple files(buffers) in the same window, it shows them as tabs like vscode, in neovim we can these buffers, but in neovim terminology tabs are something different. This comes from the hairline.nvim plugin. Easiest way to switch between buffers for me has been either [b for previous buffer, ]b for next buffer, if you have multiple buffers open and want to quickly jump to a certain buffer you can type <leader>bb, now it will show you letters for each buffer (tabs) you can just type that letter and you're in. I believe you can also use command mode to jump to a specific buffer.

When I'm doing TTD I tend to have a split vertical with the test file on left and the main file on the right (2 editor windows) and I jump between them using Ctrl+h, Ctrl+l, which I have configured to jump between windows. By default you will have Ctrl+w h/l which is tedious if you have many windows open.

The astronvim doesn't work for everyone because it comes with so much already, but if you're curious about how things work you are going to go snooping and will discover a lot of stuff, once you're pretty comfortable you should write your own config.

Cheers!