All Anglos are 20 buttons plus a few others. That is, the core of all Anglos is the basic 20 buttons: 2 rows of 10 buttons, each row in a version of Richter tuning. Think of it as 2 harmonicas, a fifth apart, strapped together and operated by a bellows.
A 30 button Anglo is those 20 buttons plus 10 more, and some of those 10 extras are the same notes in the opposite bellows direction. This may make fingering more convenient, and/or it may open up new harmonic possibilities, but if a note is a duplicate, that means that the actual note is already there somewhere in the standard 20b layout.
I said "a version of" Richter tuning because there is some slight variation, mainly on the pull note on the first button (lowest) on the left hand on the inside (nearest) row. The G row on a C/G, for example. I own 3 Anglos and (adjusted for key) that note is different on each of them.
Folk music and well known melodies often have no accidentals at all. Therefore you can play a lot of melodies along one row. 7 of the 8 notes are also available on the other row.
So on a CG 20 button:
On the C row you have C D E F G A B C etc.
On the G row you have G A B C D E F# G etc.
The only difference in available notes is F/F#
So, if you play in C, you have one accidental available: F#. This is the most likely accidental you will need in folk music because a lot of tunes modulate up a fifth for a few bars.
If you play in G, you have one accidental available: F (natural). This is possibly the 2nd most likely accidental you will need in folk music because some tunes modulate down a 4th for a few bars.
Also, some folk tunes have A and B parts that are a 5th apart.
This means that if you play to the keys of the instrument (C or G on a C/G box) rather than sticking to whatever key the tune is usually played in, you can play hundreds of tunes on a 20 b Anglo.
The problem is playing with other musicians. In English folk sessions, most tunes are conventionally played in G or D because of the ubiquitous D/G melodeon. In other folk traditions, such as Irish, A and E are also common. This does not mean that the tunes must be played in those keys; it only means that they usually are.
If you look at written arrangements for 30 b Anglo, you may find that they are written in, say, D to be played on a C/G box. These arrangements will usually require one or more accidentals from the extra row that you don't have on your 20b. However, it will often be possible to pick up the tunes by ear and fit them to the 20 b. It's a question of how you approach it. Some tunes will fit better in one key and some in the other.
(All the way through this, I have assumed a major key. Of course in folk music, many tunes are in related modes. For example, it is common for a tune "on the C row" to start and finish on D. Look up modes if you don't already know about this.)
Some good starting tunes for getting confident on a 20 button include:
Oh Susannah, When the Saints, Donkey Riding, British Grenadier, Waltzing Matilda, Red River Valley, English Country Gardens.
You probably know many of these well enough to whistle or hum so you should be able to find them on the Anglo. Experiment with different fingerings rather than just playing along the rows. Crossing the rows may make fingering easier, and it may allow different harmonies.
There's a lot you can do on 20 b if you think of it as an instrument in its own right, rather than "just a 20 b".
Here's an example of quite a complex harmonic arrangement in G on a 20 b C/G:
Here's an example of a tune played "just as a tune" in C on a C/G.