Yes, the TV app > Settings > File puts a TV library in ~/Movies by default and allows you to change this location.
However, I can’t find way to change the location of the Books library, which apparently is (or used to be) in ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBook/Books
Perhaps @james.cutler knows the Books library default location and how to change it in macOS 15.2.
James, how did you manage to create symlinks for ~/Movies, ~/Music, and ~/Pictures?
Did you do it from macOS Recovery? Turn off System Integrity Protection (SIP)?
UPDATE January 17, 2025
I restarted in macOS Recovery—no need to turn off SIP—and was able to use Terminal to delete ~/Movies, ~/Music, and ~/Pictures.
Then I restarted and logged into my user and created the symbolic links successfully:
nello@miniMe ~ % pwd
/Users/nello
nello@miniMe ~ % ls -l@e
total 0
drwx------@ 4 nello staff 128 Jan 16 20:40 Desktop
com.apple.macl 72
0: group:everyone deny delete
lrwxr-xr-x 1 nello staff 38 Jan 16 19:17 Documents -> /Volumes/Sidecar/Users/nello/Documents
lrwxr-xr-x 1 nello staff 38 Jan 16 19:21 Downloads -> /Volumes/Sidecar/Users/nello/Downloads
drwx------+ 85 nello staff 2720 Jan 16 21:30 Library
0: group:everyone deny delete
lrwxr-xr-x 1 nello staff 35 Jan 17 13:13 Movies -> /Volumes/Sidecar/Users/nello/Movies
lrwxr-xr-x 1 nello staff 34 Jan 17 13:13 Music -> /Volumes/Sidecar/Users/nello/Music
lrwxr-xr-x 1 nello staff 37 Jan 17 13:14 Pictures -> /Volumes/Sidecar/Users/nello/Pictures
lrwxr-xr-x 1 nello staff 35 Jan 16 19:26 Public -> /Volumes/Sidecar/Users/nello/Public
And, yes, I can do a cold boot, log into my user (on the internal drive), and the encrypted external drive is automatically mounted. (Yes, the first time a dialog popped up asking for the external drive’s encryption key; I was able to log into my password manager, copy the key from there, and paste it into the dialog box and check the box to store it on my Keychain. Since then, external drive mounts without any interaction on my part, presumably using the encryption key from my Keychain. More precisely, there is a brief pause in the login progress bar and then it seems to start all over, perhaps after the external drive is mounted.)
Yippee! It all seems to work!
UPDATE Wednesday, January 22, 2025 8:17 PM
AirDrop won’t work correctly if ~/Downloads is a symlink; it must be a directory.