"If yes, how can employees contribute 23k + after tax contributions, which I know many of my clients do."

That's what I pointed out. It is possible those after-tax contributions are not allowed"

These after-tax 401K contributions are allowed. They're from various employers that allow their employees to max out elective deferrals  and contribute after-tax (which are then converted to Roth). Therefore I don't think the elective deferral limit applies to the after-tax contributions. Now is the limit per person or per plan? Can my client, who switched employers and has reached the $69K limit from the first employer,  contribute after-tax 401K with the new employer? I read the links you provided, they just don't answer my question. Another excerpt from the IRS link. I think the answer is yes.

"Remember that annual contributions to all of your accounts maintained by one employer (and any related employer) - this includes elective deferrals, employee contributions, employer matching and discretionary contributions and allocations of forfeitures, to your accounts, but not including catch-up contributions - may not exceed the lesser of 100% of your compensation or $69,000 for 2024"