http://hjsplit.org/ ,there is more info on the faq link https://animetosho.org/about .You just need to run the program ,select join and click the first file in the list (.001)of links you have downloaded in the input section, then leave it to run till 100% completion .

In this case, you do not need 7-zip to get the movie.
Download the program that anon mentions above, select "join" out of the menu followed by "input file" then find the .mp4.001
hjsplit will then combine all part files it can find.

If you don't want to use a program:

Windows:

COPY /B <source file1>+<source file2>+....+<source file7> <destination file>

where <source file1> is Detective Conan - Movie 6 - The Phantom of Baker Street [DCTP][1080p][H264][D9B9B401].mp4.001
and all the other ones are the next pieces.

Linux:

cat file1 file2 > file3

Hope this helps!

Suggestions:
1. The version of 7-zip has to be 9.2 or later or it won't work
2. Create a new folder on the root of your drive with a short name, like c:\Conan.  Move all the parts there and try 7-zip again.  Windows has a limit on the number of characters in a file name plus it's path.  When it gets near that limit, things don't work but give no reason why.  And this is a long file name.  After joining, the file will probably work wherever you usually keep these shows.  (It's usually the temp files that get too long.  The full address limit used to be 255 characters.  It probably still is, to retain useless backward compatibility with machines older than most of us.)
3. Maybe there was something wrong just with the first part.  Download it again, or get part one from another service that has the 7 parts.  Admin was making these parts interchangable, he said.  Bayfiles still has it.
4. During download, some servers create hard spaces between words (blank_underline_space) instead of soft spaces.  Make sure all seven parts of the title are exactly identical except for the 001 - 007 at the end of each.  Sort by name in the folder so the numbers are in the right order before using 7-zip.

That's all I can think of right now.

To answer your question, I think admin uses 7-zip to create all archives.

That would do it.  Re-name.  What look like small differences to humans are just expected or unexpected values to a computer.

-----------------------
I just started a putlocker download of part 001.  Here's the filename I'm getting:

%27Detective%20Conan%20-%20Movie%206%20-%20The%20Phantom%20of%20Baker%20Street%20[DCTP][1080p][H264][D9B9B401].mp4.001%27

Did you get anything like that?  These are format codes for underlining, hard spacing and special fonts or characters.  To make this work, you have to re-name each of the files to match the file name listed on the putlocker screen.  In this case,

Detective Conan - Movie 6 - The Phantom of Baker Street [DCTP][1080p][H264][D9B9B401].mp4.001

Not hard, just use the mouse to copy then right click on the file, choose "Re-name" and ctrl-v to paste.  You'll have to do this or whatever for each file so they match the clean example above, except for the end number.  Note: even an extra soft space somewhere in one of them will screw things up.

Usually, when a service does this sort of thing it shows up on the download page.  This one doesn't, so it wasn't my first guess for the problem.

Two last things:
1. you might be able to get away with just renaming the files to xxx.001 to xxx.007.  I forget if that works.
2. These kinds of programs used to not like "double-dot" formats in the file names.  If it still doesn't work, try removing the .mp4 part from each file.  Prefix/Suffix were what the dots were supposed to be for originally -Past and present rules blur for me sometimes.