I'm reading these notes

where it states in section $3$: (transcribed because I can't post image)

Step 1.

Introduce the stacks of degenerated and iterated shtukas which extends that of shtukas.This step is based on the well-studied scheme of complete homomorphisms of rank $r$ which is obtained from the scheme of non-zero $n \times n$ matrices by a series of blow-ups. Roughly speaking, the last condition $E^σ \xrightarrow{\sim} E^{''}$ will be replaced by a complete homomorphism $E^{σ} ⇒ E^{''}$ .

Referring to the construction of some kind of compactification of the stack of Drinfeld's Shtukas. What is this "well-studied scheme of morphisms via blowups" and what are any references? I don't even know what to google for a reference to this.