using _< instead of < for stdin using bash

That's not a _< operator, that's a _ argument to pass to read and the < redirection operator. <(cmd) itself is process subtitution (that expands to a filename that points to a pipe).

What that does is run:

read bytes _  < /proc/self/fd/x

Where the fd x is the reading end of a pipe.

At the other (writing) end of the pipe, a background subshell process is executing du -bcm random_iso.iso | tail -1 with its stdout redirected to that pipe.

So read will store in the $bytes variable the first word of the last line of the output of du -bcm, and the rest of the line in the $_ variable.

Now I don't know where that du -bcm makes sense. None of -b, -c nor -m options are standard. While -c is quite common and is for giving the cumulative size, with GNU du, -b is to get the file size (not disk usage) in bytes, while -m is to get the size rounded up to the next mebibyte so they would be conflicting options (though maybe they used -b for its side-effect of enabling --apparent-size). FreeBSD du has -m (for mebibytes), no -b, Solaris has neither...

It looks like it was meant as a convoluted way to write:

wc -c < random_iso.iso

Or:

du --apparent-size -cm random_iso.iso | awk 'END{print $1}'

If they indeed wanted the file size rounded up to the next mebibyte on a GNU system.