After you file

I presume you mean why does your efiled return status show "pending."   The IRS will open its season today for processing of 2023 returns.  A few returns were processing early for test purposes.   But for most people, their returns will start processing today (Monday, 1/29/2024).   However, there will be a huge backlog on opening day, so you may or may not receive word about "acceptance/rejection" today.