Startup initial sync with different files

I have on PC A and PC B many, many MP3 files. But they are not in sync right now. The directory structure is similar on both PCs, but some files are missing on PC A and some on PC B… Some files are newer on one PC (e.g. added MP3tags)

How do i start the syncthing here?

What happen if i just add the reprository on both PC?

Should i use e.g. unioson to merge the files first?

You could create two repositories, let them sync. Then move all files from one repository to the other. Then let it sync and remove the empty one.

If you just add the repo on both nodes, syncthing will merge the files (those missing on A will be added on B and vice versa). However if you have differing versions of files, (i.e. a.txt with different contents on both sides) one will be overwritten. If you care, sync the repositories with rsync or something before you start, or do as @jpjp suggests.

I will have files with differerent content. By the sync, is should overwrite the old file with the newer one, isn’t it?

I had thought, that missing files on A can be seens as “deleted file” so that they will be deleted on B.

How does deletions will be merges? Only file that was in the index and then removed/deleted will be also deleted on the other side? And there is currently no Backup, like rsync’s –backup-dir= isn’t it?