I tried to change the send order on a track in reaper; to put the reverb send before the delay but found out its not possible to switch the initial order. Does the order of sends on a track affects it or does the track treats each send separately?
TL;DR: sends are treated in parallel, so there is no concept of order as opposed to FX chains (inserts) which treat the audio signal in series.
Unfortunately I don't have a schema ready to show you, but I'll explain with words as best as possible, hoping I understood your question properly.
I could describe a very generic model of audio processing, made of processing (or "filter" to use a signal processing term) nodes having audio inputs and audio outputs. For the sake of simplicity, I will get closer to the reality, especially in Reaper, using the same terms.
Consider a track as a black box having audio inputs and audio outputs. You can have as many inputs and outputs as you want, and they are numbered starting from 1. Each input or output is called a channel. For instance, when you create a classic send from one track to another, you will use the output channels 1 and 2 from the source track, and connect them respectively to the input channels 1 and 2 of the destination track.
What happens with a track's FX chain though remains internal to the track (the box). Input channels are redirected to the FX chain, the FX chain processes audio in series, and the chain redirects back the audio to the output channels of the track. Since processing is done in series the order is indeed important.
However, when using sends/receives between tracks, you actually connect output channels of a track to input channels of another. When you create multiple connections using the same output channels or input channels, processing is done in parallel. In other words, two sends from the same output channels of a track will send the exact same audio signal to the two destinations, creating two independent audio connections (it splits the audio). Since they are parallel, the order doesn't matter. For information, two receives into the same input channels of a track will mix the two input audio signals (it merges the audio).
This was a brief introduction to audio routing applied to Reaper, and I didn't cover subtleties such as cyclic routing (which I think is just forbidden), mixing algorithm (to merge audio into input channels), latency compensation (for routing through multiple tracks in parallel), etc. In the case of your question, I already explained far more than necessary.