Derivation of magnetising force on the axis of a short solenoid

Question

Reference I will refer to is Textbook of Electrical Technology by Theraja, page 268 (iv) Magnetising Force on the Axis of a Short Solenoid

The derivation is already there. I just don't understand the visuals that well because they look so complicated. I don't get the use of the variables clearly such as the θ, dx, θ1, θ2, r, and l and even the points M, N and P. Recently I have already studied and understood (iii) Magnetising Force on the Axis of a Circular Coil. But what I don't get is why they have to use that on this part even though this is about a short solenoid. It's not even circular. I am already lost with the whole, use the circular coil thing. I understand the idea of using differentials to represent infinitesimal values but the situation this time feels so complicated.

I mean, the derivation is already there and pretty clear for someone else who probably understands the topic. I just want a more clear explanation, especially on the visuals and how it relates to the Circular Coil geometry.