Reference Request: Compact subgroups of p-adic Reductive Groups

Question

First, I'd like to understand what the compact open subgroups of an inner form of $G := GL_n(\mathbb{Q}_p)$ are, and how they relate to the compact open subgroups of $G$.

Second, I'd like to know where I can read about this for other reductive groups $G$.

Any pointers would be greatly appreciated. Thanks!