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Buff/cache means cached pages from disk into memory for higher system speed. It is cache of contents from disk loaded into RAM. When program requests reading of file already in cache, its contents is served from RAM. It does not have to wait each time until it is read from disk. Loading times for SSD are close to acessing RAM, so this will not make huge difference. Available is described in manual page:

          Estimation of how much memory  is  available  for  starting  new
          applications,  without swapping. Unlike the data provided by the
          cache or free fields, this field takes into account  page  cache
          and also that not all reclaimable memory slabs will be reclaimed
          due to items being in use (MemAvailable in /proc/meminfo, avail‐
          able on kernels 3.14, emulated on kernels 2.6.27+, otherwise the
          same as free)

Difference between free and available is that free is not used for anything useful. Available includes cache parts, that can be freed if more memory is required. Buff will fill with IO operations of running system. It will load contents into RAM for higher speed. Because your server is more busy and longer running than your desktop, it already has full cache. That is ok, free memory is wasted memory.