There is no separate category maximum for employee contributions, but they are part of the total annual additions limit of 57,000 that applies to all contributions to the plan including company match. Sometimes the plan will limit after tax contributions so that the employee does not come near to the max or to limit discrimination testing failures.
If an after tax contribution is rolled to a Roth IRA after earning $1, the taxable amount will be that one dollar. For the balance in the after tax sub account, if there is on line access for the account, it should show up there. Otherwise, a call would be necessary.
The plan provisions will state the frequency of rollovers allowed. There is no IRS limit. Many plans do NOT offer “employee contributions”, again this is a plan option that is typically only used by higher income staff. More plans offer a designated Roth account, but some of those do not offer in plan Roth rollovers. Some plans that do offer in plan Roth rollovers require that the after tax sub account be rolled there instead of to a Roth IRA.
For distributions from the after tax account, a 1099R will be issued showing the taxable amount, and that is reported on Form 1040.
401k balances are required to be kept in separate accounts for each employee and cannot be used for any company purposes. These accounts are safe even if the company goes under.