Do these all work?
1. get into the job through/by the backdoor.
"through the backdoor", not by.
"How did he get hired?"
"Oh, they brought him in through the backdoor; his dad is the boss."
2. backdoor entry into a company
3. backdoor entrance into a company
I'm also wondering what are the cases of
e.g. an influential politician's son
getting a job without taking entrance exams?
Through the backdoor is the only use I am familiar with.
"He did not have to take the entrance test, he got in through the back door because his dad is the mayor."
"Under the table job"... a job where the company does not tell the government you have a job; you do not pay taxes, have health benefits, you are paid with cash, not a check, etc.
Companies hire people under the table when they want to save money on taxes. It is illegal.
"We want you to work for us for a month, but we can only hire you under the table."
"No thanks, I need to have health insurance, so I need a regular job."
"I can fix your house for $500, but it will be only $300 if you pay me under the table in cash, not by check (so I do not have to pay taxes on it.)
Student or Learner