AS/400 isn't a mainframe, it's IBM's minicomputer arch.
Naming details,which I always get wrong, because nowadays PC servers have taken that role.
It's just weird to bring up AS/400 in a conversation about mainframes.
It is as far away from a x86 server than a s/390 was from an AS/400. It was a unique architecture that came from, IIRC, the System/38. Also, its native encoding is EBCDIC. It shares a couple concepts and names, such as LPARs.
It has stuff like LPARs because it shares a processor arch with IBM's COTS servers running POWER (yes it has an extension to have tagged pointers, but they demoed it originally on unmodified POWER).
Ie. it shares way more with regular server boxes than mainframes.
Had to perform regular backups on the system as part of my internship.
Nowadays kids are all up with WebAssembly and WASI, well that is just how OS/400 works during the last 30+ years.
Originally designed in a mix of PL/S and Assembly, everything else (RPG, Cobol, PL/I, C, C++) compiled into ILE (Intermediate Language Environment) and cross language calls are relatively easy to do.
ILE applications are AOT compiled either at installation time, or anytime some critical hardware has changed or the application themselves have been updated.
Nowadays there is also Metal C (real native not ILE), Java via IBM's own JVM (which in early versions converted JVM bytecodes into ILE ones), and the C and C++ compilers are also to target actual native code besides ILE.
The database backed filesystem took some time getting used to, coming from Amiga/MS-DOS/Windows 3.x/Xenix experience, and the command line felt more cryptic those OSes, given the use of special characters as part of the name.
The company I did my internship was using them for their accounting, everything else were MS-DOS/Windows computers connected via Novel Netware, zero UNIX flavour in sight.
IBM z and Unisys ClearPath are two other mainframe models that also follow similar bytecode based deployment formats.
So in a sense you can say that on every Android phone, watchOS or Windows PC lives a little mainframe.