Where in the database file is an index rebuild started? Alternative to SHRINKFILE? When rebuilding an index in SQL server, where in the datafile does it put that data?

Does it start at the beginning of free space and fill in the gaps from there?

Context:  I have some large .ndf files I'm trying to shrink (not important why); but instead of using a SHRINKFILE command, I'm wondering if I can rebuild the indexes and then truncate the file from 80GB down to 25GB.

Edit:  There are many reasons to want to reduce the file space or reorganize it logically.  These may relate to fragmentation, how that fragmentation is translated to IO requests, moving large files across a network, swapping out storage space, how data is read sequentially, etc.  Excessive space usage could even prevent other files from being written to your disk that need the space or dbs that need room to grow.  You might have done some testing and now need to clean up excessive data and the space it required.  Imagine you just switched to a newer database server and could take a 400GB database and compress it down to 100GB with newly available db management techniques, compression, etc,  That's got value and I'm sure there are many other scenarios where this matters.  Just be careful to judge the reasons behind questions please.
I don't believe those details are documented anywhere, and even if you were to figure it out (by asking Paul Randal, or perhaps by mapping the exact pages before and after an index rebuild), you may not be able to rely on those going forward. You also might end up with some random single pages at the end of the file anyway, and still need to do a shrink.

The best idea, in my opinion, is to do an index rebuild into a different file group:


  
  Create a new filegroup
  Move all affected tables and indexes into the new filegroup using the CREATE INDEX … WITH (DROP_EXISTING = ON) ON syntax, to move the tables and remove fragmentation from them at the same time
  Drop the old filegroup that you were going to shrink anyway (or shrink it way down if its the primary filegroup)
  


If that's not an option, then you can instead do a shrink-then-reindex, or maybe a reindex-shrink-reindex:


Initial reindex: this releases free space within each page, and might be necessary if you've made major changes in the table structure like dropping large columns or changing column datatypes
ShrinkFile (never use ShrinkDatabase): Pick a reasonable target size, shrink each file with some remaining free space (I usually pick 15-20% free)
Final reindex: Resolves any fragmentation we created in step 2. 


This multi-pass approach is a ton of IO, and might take a while to do. That's why the single-pass approach into a different file group is preferred, if you can pre-allocate the space.

Also, the final reindex may reclaim some of the space recovered in step 2. This is common where the majority of space is taken up by a single huge table or single huge index.