Hey guys. I am new to the site and was wondering if someone could help me out with some questions on theory. I was looking over a site with lessons discussing modal theory. The lesson went something like this: Key of G

G Ionian
A Dorian
B Phrygian
C Lydian
D Mixolydian
E Aeolian
F# Locrian
If I want to play something in G major I can use the major(ionian) scale of
G-A-B-C-D-E-F#-G and the chords that I could choose for the rhythm are G(I) Am(II) Bm(III) C(IV) D(V) Em(VI) and F#(VII).
This all sounds okay so far, right?

Then he goes on to say that if I wanted to play in a minor, I could go to the second mode, the A Dorian. He said that ALL the notes in the scale would remain the SAME but I would simply start AND end on the A note instead of the G. Is it really that simple?
Here is where he confused methough. Two pages later he breaks down the differences in the modes with a chart as follows.
IONIAN 1-2-3-4-5-6-7-1
DORIAN 1-2-b3-4-5-6-b7-1
and he says to get the dorian just take the ionian and flat 3rd and 7th

So first he tells me the Dorian would be A-B-C-D-E-F#-G-A.
Then if I understood his chart, the Dorian would be G-A-A#-C-D-E-F-G
Can anyone straighten me out on this?
Also, would the available chords change in going to the Dorian?
This is my first post so hopefully someone can help. Thanks.