The thumb called (P or T) takes charge of the lowest three strings on the guitar (ide the E, A, and D strings).
Whereas the first finger (the index finger) also referred to as i or 1 will mostly play the G string, the middle finger/the Second finger (which is notated as m or 2) will handle the B string and finally, the 3rd finger (ring finger) will play the high e strings.
Again, these are starting points, but we can always adapt our fingering to what's in front of us.
The patterns displayed in this lesson will mainly use the C, Am, G, and/or G7 and the F chords.
You do not have to adopt the same chords if you think they're too easy or too hard, as the principles will work for any chord.
Remember to use some of these patterns to create interesting musical variations in your guitar playing.
For instance, let's consider you are covering a song or playing one of your original songs. One way of using those patterns is to start the verse with Pattern 1 (or any pattern for that matter) but then switch to Pattern #2 for the chorus (or any other pattern).
Another way is to switch between strumming chords and arpeggiating chords.
P.S: here is another lesson for you that will help you master the basics of fingerpicking!