Understanding expected TESS data structure
An insightful option is to open in a python IDE using Astropy. I find it useful because you can manipulate the files as your learn their structure.

It is also possible to use matplotlib in conjunction with Astropy and then basically have a FITS file viewer that you can code yourself.

Something like:

import astropy
import matplotlib.pyplot as plt
from astropy.visualization import astropy_mpl_style


    def open_fits_file(fits_path):
        for file in Path(fits_path).iterdir():
            #Not really using the with, but it'll manage closing the file...
            with fits.open(file) as opened_fits:
                #printing header infos for that file
                opened_fits.info()
                plot_image(fits.getdata(file, ext=2))


Then to plot the image:

def plot_image(fit_image_data):
    #astropy has better plot settings for this use
    plt.style.use(astropy_mpl_style)
    plt.figure()
    plt.imshow(fit_image_data, cmap="gray")
    plt.colorbar()
    plt.show()


I used that to plot the pixels  from these simulated data linked in the question.

As mentioned by Mike G, HEASARC as a list of viewer, including FV (fits viewer). It works fine and it has some built-in support to link up with other astronomy database. It has an apollo-era feel in terms of appearance, which is nice... I guess?