which impedance is matched? Why?

Impedance matching means that one side impedance equals or joins another impedance with a power transfer without reflection.
Your picture 1 shows a line with 50 Ohms facing two parallel 50-Ohm lines: this is a mismatch as the impedances on the left and to the right of the junction are not equal: one is 50 Ohms, the other 25 Ohms.
Fig.2 shows 50 Ohms on the left and 50 Ohms on the right , therefore this junction is matched.

Instead of lines you can consider source and load impedances.

just imagine two parallel resistor one right hand side of figure A and B and calculate equivalent value of the parallel resistor.

fig A: 50ohm in parallel with 50ohm = 25ohm so 50ohm in series with 25ohm is missmatch.

fig B: 100ohm in parallel with 100ohm = 50ohm so 50ohm in series with 50ohm is perfect match!