According to macOS Help, safe mode stops some software from loading, and performs a check of the startup disk. Here’s a more detailed and accurate account of what it does.
fsck
Don’t waste time trying to discover which files are still open when you want to check or repair an APFS disk and Disk Utility throws an error. Here’s the solution.
When your backups are stored on hard disks, even though they may use APFS, it’s important to check they’re sound. How to do better than Disk Utility.
APFS is almost 4 years old, and set out to solve the problem of accumulation of minor errors which has plagued HFS+. Has it?
Why you should always protect backup storage with a UPS. Should you also rebuild its directories every month? And when to archive rather than back up.
Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
Disk integrity checks appear to have been dropped from Safe Boot. OpenDirectory databases are rebuilt. With a full listing of all blocked extensions.
Start up in Safe Mode: should it start by checking and repairing your boot disk, or should you do that separately in Recovery Mode?
Third party tools for checking and repairing APFS are still very limited. Is it better to use fsck to check and repair APFS?
Single-user mode to run fsck to check and repair your boot disk became tricky with Fusion Drives. With a T2 chip, it’s not practical in its traditional form.
The Eclectic Light Company 