The $58,000 limit refers to the maximum employee and employer contribution. As an employee you are limited to $19,500. See Example 1 in Retirement Topics - 401(k) and Profit-Sharing Plan Contribution Limits
The remainder is a non-elective contributions made by the employer or commonly to a profit-sharing plan by an individual with a Solo 401(k) plan. See Ben example in One-Participant 401(k) Plans
What is a Non-Elective Contribution?
