Doctors do not get a kick back for sending you off to Specialist.

The problem with Medicare is , yes many Doctors are opting out of accepting Medicare because there are receiving such low payments. They will see you but you pay for the visit then you file your claim to medicare and they will reimburse you for the allowable amount.( same as what they would directly pay your Doctor)

Normally: You Doctor sees you and his/her charges 105.00 for the visit. They file the claim.
Medicare drops down the amount the Doctor can bill for.
so Medicare slashes the "allowed amount to say 75.00 then they only pay 80% of the "allowable charge".
You are left with that 20% owed by you.

If you are of age to purchase a secondary insurance that will cover your 20% That's fantastic.
If your not of age to purchase such a plan,( as I am) Yep 20% is on your shoulders to pay.

Part A covers hospital
Part B covers Doctor visits
Part D is what covers your medication costs.

Of course you have to first pay your yearly deductable before actually receiving any monies paid to your Doctor(s)by medicare.

Of course Medicare only pays 65% of mental health visits, ( if this applies to you)

Medical information and coverage is a very difficult thing to understand , I think its done that way on purpose.