Em7b5 is built from E, G, Bb and D. E is the root, G the minor third, Bb the flat fifth and D the minor seventh. Said aloud, its notes are E, G, B flat and D.
String by string, low to high, it plays E, Bb, E, G, D and E. E rings three times across the six strings, and it is the root. The lowest and highest notes are both E, two octaves apart, on strings 6 and 1.
The open strings add notes to Em7b5 too. String 6 gives E, string 3 gives G and string 1 gives E. Your hand covers three frets, from 1 to 3. There is one more position, starting at fret 7.
Em is very close. You only move strings 5 and 2. Inside Em7b5 sits a complete Gm chord, made of G, Bb and D. Moving to C, you keep one finger down.
Em7b5 shows up in two keys. In F major it is the viiø7 and in D minor the iiø7.