SSDs have their own logical to physical mapping tables. When you trim, it is telling the SSD you no longer care about that entry in the table so the physical space can be freed up. In terms of latency there is no seek time difference from one location to another hence defrag has no benefit. In fact defrag will just cause more wear.
Fragmentation is even beneficial in SSDs. When the contents of a logical file are physically scattered, the internal parallelism of many flash devices can be exploited. If your file are physically compact, you'd lose that parallelism.
I used to work on SSD FTL so have some ideas on the internals. Generally the writes are stripped across the flash devices. Main thing is you need command queueing to exploit the parallelism. You can do a few tricks like stride detection to prefetch in some cases because it makes the benchmarks look good.
