Many practitioners who don’t pay Bloomberg, CCH/Wolters Kluwer, Lexis, Tax Analysts, Thomson Reuters/Westlaw, or another commercial publisher for a professionally edited and annotated version, use:
Bloomberg’s free version https://irc.bloombergtax.com/;
or the US Government’s United States Code. http://uscode.house.gov/.
In that compilation, I.R.C. § 401 = 26 U.S.C. § 401.
The government’s compilation includes SECURE 2022 provisions applicable for 2024, and shows full sets of amendment notes.
Bloomberg’s version (even its free version) displays in-context notes to show which provisions apply to 2024, which apply to earlier years, and which not-yet-applicable provisions will apply for later years—2025, 2026, 2027, 2028, 2029, and 2030.