You have worked for several "airlines" and still don't understand the regulations under which you flew??
A SIC is required for carrying passengers under IFR under Part 135. If an operator wishes to use an autopilot in lieu of a SIC, then the operator needs an operational specification authorization to do so, found in OpSpec A015.
The requirement for a SIC is found in 14 CFR 135.101:
§ 135.101 Second in command required under IFR.
Except as provided in
§ 135.105, no person may operate an aircraft carrying passengers under IFR unless there is a second in command in the aircraft.
https://www.ecfr.gov/current/title-1...ection-135.101
You will notice that an exception is provided in 135.105, which is that an autopilot may be used in lieu of a SIC. This is permissible if approved in the Operations Specifications (OpSpecs) issued by the FAA to that certificate holder.
There is no OpSpec requiring a SIC: 135.101 requires the SIC. An OpSpec is required to operate under IFR without the SIC. A second in command may be required by the aircraft type certification, or the rules under which the aircraft is operated. In this case, operations under IFR under Part 135, require a SIC, regardless of whether the aircraft is type certificated for one pilot or not. To get around this requirement, the operator must hold OpSpec A015. I'm surprised that you operated single pilot IFR operations for "three airlines" under Part 135 and do not fully understand this.
§ 135.105 Exception to second in command requirement: Approval for use of autopilot system.
(a) Except as provided in
§§ 135.99 and
135.111, unless two pilots are required by this chapter for operations under VFR, a person may operate an aircraft without a second in command, if it is equipped with an operative approved autopilot system and the use of that system is authorized by appropriate operations specifications. No certificate holder may use any person, nor may any person serve, as a pilot in command under this section of an aircraft operated in a commuter operation, as defined in
part 119 of this chapter unless that person has at least 100 hours pilot in command flight time in the make and model of aircraft to be flown and has met all other applicable requirements of this part.
(b) The certificate holder may apply for an amendment of its operations specifications to authorize the use of an autopilot system in place of a second in command.
(c) The Administrator issues an amendment to the operations specifications authorizing the use of an autopilot system, in place of a second in command, if -
(1) The autopilot is capable of operating the aircraft controls to maintain flight and maneuver it about the three axes; and
(2) The certificate holder shows, to the satisfaction of the Administrator, that operations using the autopilot system can be conducted safely and in compliance with this part.
The amendment contains any conditions or limitations on the use of the autopilot system that the Administrator determines are needed in the interest of safety.
https://www.ecfr.gov/current/title-1...ection-135.105