I see that in Renaissance music printings, the noteheads are diamond and square shaped as opposed to circular, and the stem for each note comes up or down from the middle instead of from one side (I'll link a few pictures below showing what I'm talking about). Did they really write their music down like this? Or is this style just the way the music was printed, as opposed to how it was written down initially by the composer? It seems rather difficult and tedious to make the diamond and square shapes for every note, especially with the quill pens that I assume they used. Also, were there any specific tools (ink, writing utencil, paper, etc.) that they used to write music? Or was my assumption that they just use a standard ink and quill correct? I'm curious about how Renaissance composers actually wrote their music down.
Yes, they really wrote like this, just as much as people today write (or sometimes do not write) letters like V or K with straight strokes and sharp angles. The printing style is based on the handwriting style, not the other way around. See for example this manuscript from the early 1500s:
Remember, the paper they were writing on was not necessarily as smooth as today's paper, and the nibs they used were quite broad, so it was not necessarily as easy to make a circular stroke as you might think from your own experience using a pen with a round steel or felt tip on smooth modern paper.
For an example of a rounder style, see this English manuscript from the middle of the 1500s:
The reasons for white notes may be found in this translation of the German Wiki article MENSURAL NOTATION:
Before the invention of printing, choirs usually only had a single handwritten copy of a work at their disposal. Due to the enlargement of the choirs, the notes were written larger and larger so that every singer could read from the choir book. For the sake of simplicity, only the outlines of the notes were drawn, creating white, "hollow" notes (just as half and whole notes are hollow in today's notation). Another reason for the transition to white mensural notation is the replacement of thick parchment with thin paper in the 15th century, because filled notes often shone through on paper.
Originally, all notes were written in solid, filled-in form ("black notation"). In the mid-15th century, scribes began to use hollow note shapes ("white notation"), reserving black shapes only for the smallest note values. This change was probably motivated by the change from parchment to paper as the most common writing material, as paper was less suited to holding large dots of ink.