Ignoring the interest payments, the repayment of the loan is simply putting back the money that you took out. It's not treated as either pre-tax or after tax unless you default on the loan, in which case the loan becomes a deemed distribution that is taxable in the same pro-rata manner as would be an ordinary distribution. Because the amount of the deemed distribution will now have been fully taxed, subsequent repayments of the loan that had been declared to be a deemed distribution will become after-tax basis in the 401(k).
Also, after-tax contributions, Roth contributions and pre-tax contributions and their attributable earnings are all tracked with separate, independent accounting. This separate accounting effectively makes them separate sub-accounts, not treated as a single account. The after-tax sub-account (with its attributable earnings) can be distributed separately from the others, so the pro-rata calculation of the taxable amount of a distribution from the after-tax sub-account would be based only on the funds in the after-tax account. This is why the mega-backdoor-Roth works.
IRS Rev. Proc. 2020-46 allows a for single distribution to be split and rolled over to two separate accounts with the taxable portion of distribution going to a traditional IRA and the after-tax portion going to a Roth IRA. If not done as a single distribution, the pro-rata calculation would mean that a rollover to a traditional IRA would result in some portion of that distribution being after-tax money that becomes nondeductible contributions in the traditional IRA and some portion of the separate rollover to the Roth IRA being taxable.
