I looked it up and it seems to me that the limit for after-tax Roth is per employer, not per person. So wanted to confirm if others interpretations is the same as mine.

"Total annual contributions (annual additions) to all of your accounts in plans maintained by one employer (and any related employer) are limited. "

Example 1: In 2020, Greg, 46, is employed by an employer with a 401(k) plan, and he also works as an independent contractor for an unrelated business and sets up a solo 401(k). Greg contributes the maximum amount to his employer’s 401(k) plan for 2020, $19,500. He would also like to contribute the maximum amount to his solo 401(k) plan. He is not able to make further elective deferrals to his solo 401(k) plan because he has already contributed his personal maximum, $19,500. He would also like to contribute the maximum amount to his solo 401(k) plan.

Greg is not able to make further elective salary deferrals to his solo 401(k) plan because he has already contributed his personal maximum, $19,500, to his employer’s plan. However, he has enough earned income from his business to contribute the overall maximum for the year, $57,000. Greg can make a nonelective contribution of $57,000 to his solo 401(k) plan. This $57,000 limit is not reduced by the elective deferrals Greg made under his employer’s plan because the limit on annual additions applies to each plan separately.

https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-401k-and-profit-sha...