How do I modify command options to have "addr" as variable?

QP->start("/bin/sh", { "-c", "echo q  | sudo -S hcitool info " + _ " addr" })

I'm not sure what the underscore is doing in there. Maybe a typo?

Why on earth would you think that addr inside a quoted string would refer to a program variable? If you had a variable info or echo in your program, would you expect the values of those to be substituted into the string as well????

Assuming std::string semantics for the arguments are for QP->start, then you should be able to use

QP->start("/bin/sh", { "-c", "echo q  | sudo -S hcitool info " + addr });

If you the arguments are C style NUL terminated strings, then you've got options:

// using std::string
std::string cmdstr{"echo q  | sudo -S hcitool info ");
cmdstr.append(addr);
QP->start("/bin/sh", { "-c", cmdstr.c_str() };

//using sprintf
#include <cstdio>
char cmdstr[256]; // something big enough to not overflow
sprintf(cmdstr, "echo q  | sudo -S hcitool info %s", addr);
QP->start("/bin/sh", { "-c", cmdstr};

//using strcat
#include <cstring>
char cmdstr[256]{"echo q  | sudo -S hcitool info "};
strcat(cmdstr, addr);
QP->start("/bin/sh", { "-c", cmdstr};

//using a string-stream
#include <sstream>
std::stringstream command{"echo q  | sudo -S hcitool info "};
comand << addr;
QP->start("/bin/sh", { "-c", command.str().c_str() };

There's probably others as well. I'm sure QString and friends provide similar facilities.