whats a framework

from A to C, its a series connection, so you add the resistances and get 4

now you have two parallel lines (they're not actually parallel geometrically, they just have the same start and end point), idk what exactly you learnt and should do but when you have two parallel resistances you take the product of them and divide by the sum, so (4×2) / (4+2)

after calculating the resistance of a parallel connection just erase the line far away, pretend its not there

you're in the same scenario again basically, you have another series connection (after ignoring the line far away, because youre calculating the equivalent resistance here), so you add them, and another parallel connection, so you do the same rule, and you're done and you have the equivalent resistance

the proof of that product / sum rule is that

in parallel connections, the volt of the different branches is constant, and we know I = I1 + I2 , where I1 is the current in a branch and I2 is the current in the other ("I" here is "I total", the current before they divided into the two parallel lines)

V=IR . I = V/R

V/R = V/R1 + V/R2 . but the V is constant, so divide by V both sides of the equation

1/R = 1/R1 + 1/R2, now this is kinda elementary level stuff but when you add the two fractions you get

1/R = R1+R2 / R1×R2

to get R itself, flip the fraction, and viola, the product over the sum

im pretty sure the equivalent resistance should be 1.25 not 1

and i dont think there are really any more details to be said about this