Who converts binary/machine code to electrical signals and how?
In old computer you actually had a row of switches on the front panel
that would allow entering 1s and 0s directly by hand in the registers
of the machines. But this is no longer done.

You never have 1s and 0s, unless you ask the machine to print its
internal information as a sequence of 1s and 0s. All information is
already in the machine encoded as voltage, or magnetic orientation, or
hole in a physical substrate, or some other physical form. Various
devices can perform the translation between these form: magnetic
heads, laser beams, electro-mechanical devices, electronic
circuits,etc.

Some form are better adapted at memorizing or transmission by various
means, while other (electrical signal notably) are better adapted at
processing, usually by complex systems of logical gates (and, or, not,
xor ...) and micro-memories (registers).

In general, binary/machine code is no longer entered by human beings,
but produced by a program (usually a compiler) by the computer itself,
or by another computer and then imported on some memory support or by
network. However, human beings can ask to have it printed as 1s and 0s
to check what the computer is using or producing. However this
requires transforming the single bit 0 or 1 in the computer as a
stream of bits to be interpreted by the printer as a request to print
an actual drawing (on screen or paper) od the symbol representing a 0
or a 1.

It is hard to give more details without your being more specific
regarding the source form and target form of the conversion. But it is
most often from binary to binary, with a different physical
representation.