How does the aircraft keeps propelling forward when airflow and thrust are not aligned?
Let us assume level flight. Then the forces acting on the aircraft are shown in the following sketch (not necessarily to scale):

The forces are :

the total aerodynamic force $ F_A $, which is split into two components: lift $L$ (perpendicular to direction of motion) and drag $D$ (parallel to direction of motion)
the weight $W$
the thrust $T$, here acting slightly upwards because of the angle of attack $\alpha$

For un-accelerated level flight, the sum of all forces must be zero. This is independent of the coordinate system you use to split the forces. In our example, this is given by these equations:
$$ L + T \sin \alpha = W $$
$$ T \cos \alpha = D $$
At a given angle of attack, these equations will only be true at one particular airspeed. At higher airspeed, the lift would increase resulting in a climb, which then changes the angle of attack. At lower airspeed, the lift would decrease resulting in a descent.