They are always testing changes to different groups of people.
Not too sure how that works, but if you wnat the old layout you can add extensions.
Quick search results in:
How to Undo YouTube's Terrible New Layout [Jake Peterson | May 23 2024 | LifeHacker] https://lifehacker.com/tech/how-to-undo-youtubes-terrible-new-layout
How to Get Old YouTube Layout Back: Simple Steps for Nostalgic Navigation [July 18, 2024 | bytebitebit] https://bytebitebit.com/tips-tricks/how-to-get-old-youtube-layout-back/