IBM midrange is a generic term which encompasses a family of 'baby mainframe' computers which IBM have been making since the late 1960s.
While System/3, /32, /34, /36 all had flat, non-relational file systems (much like a Windows PC), System/38, AS/400, iSeries, and Power all have a midrange version of DB2 built-in. In addition to DB2, IBM i (the name of the operating system) also has a subset of AIX called PASE, a *nix-like shell called QShell, and Java built-in. The primary programming language is ILE RPG, which allows for embedded SQL as well as 'native' record level access I/O. Additionally, to retain compatibility with prior generations of the operating system, Rexx, CL (the native command scripting language), DDS (for describing database, and device files), and earlier versions of RPG are still supported. IBM i 7.2 and up includes open source software like Python, Node.js, and GCC.
Detailed instructions on getting ibm-midrange set up or installed.