Is it possible to manually TRIM the unused blocks (free space) for an existing APFS volume?
On APFS, macOS automatically performs a TRIM operation on the free disk space on boot via the AFPS Space Manager (spaceman). Therefore, as long as TRIM is enabled on the system (via trimforce for 3rd-party SSDs, see karolus's answer), you don't have to worry about it. There is no command and no need to manually trim the unused blocks.
Verification
If you want to personally verify the operation of boot time TRIM, you can search for spaceman-related messages in your boot log.
Execute the following command as root. It lists all messages from the kernel (--predicate &quot;processID == 0&quot;) since the beginning of this month (--start $(date &quot;+%Y-%m-01&quot;) and search for messages that contain &quot;spaceman&quot; (grep spaceman).
# log show --predicate &quot;processID == 0&quot; \
  --start $(date &quot;+%Y-%m-01&quot;) | grep spaceman

And you can see messages like these
2021-01-13 11:54:28.559149+0800 0x1bc      Default     0x0                  0      0    kernel: (apfs) spaceman_trim_free_blocks:3009: scan took 7.102084 s, trims took 6.415750 s
2021-01-13 11:54:28.565684+0800 0x1bc      Default     0x0                  0      0    kernel: (apfs) spaceman_trim_free_blocks:3017: 28059445 blocks free in 247937 extents
2021-01-13 11:54:28.572872+0800 0x1bc      Default     0x0                  0      0    kernel: (apfs) spaceman_trim_free_blocks:3025: 28059445 blocks trimmed in 247937 extents (25 us/trim, 38645 trims/s)
2021-01-13 11:54:28.580121+0800 0x1bc      Default     0x0                  0      0    kernel: (apfs) spaceman_trim_free_blocks:3028: trim distribution 1:25496 2+:47953 4+:92857 16+:41261 64+:17838 256+:22532

The lines are too long to read, I slightly edited it to make it shorter. You should see logs lines that mentions &quot;TRIM&quot;,
scan took 7.102084 s, trims took 6.415750 s
28059445 blocks free in 247937 extents
28059445 blocks trimmed in 247937 extents (25 us/trim, 38645 trims/s)
trim distribution 1:25496 2+:47953 4+:92857 16+:41261 64+:17838 256+:22532

As you can see, macOS just trimmed 28059445 unused blocks today. Assuming APFS uses a 4 KiB block size, it means the SSD has discarded 107 GiB of free space. And yes, it roughly matches the free space on my disk. Works as expected!