You might have a better shot looking at 1.15.2 vanilla sources since many of the 1.16 variables aren't named yet.
This is what the render method for Screen looks like in 1.15.2:
public void render(int p_render_1_, int p_render_2_, float p_render_3_) {
for(int i = 0; i < this.buttons.size(); ++i) {
this.buttons.get(i).render(p_render_1_, p_render_2_, p_render_3_);
}
}
I also find it useful to check out the github of a mod that has already implemented something similar to what you want and base your code on theirs.
Good Luck!