Nautilus 7z compression

I know p7zip package is not installed by default on Ubuntu 18.04. Still Nautilus is able to create 7z archives. How is this possible?

Nautilus has changed the mechanism of archive compression.

The list of dynamic libraries of ldd $(which nautilus) executable confirms this.
Here the most important line is below:

$ ldd /usr/bin/nautilus
...
    libarchive.so.13 => /usr/lib/x86_64-linux-gnu/libarchive.so.13
...

What is the name of its package?

$ dpkg -S /usr/lib/x86_64-linux-gnu/libarchive.so.13
libarchive13:amd64: /usr/lib/x86_64-linux-gnu/libarchive.so.13

What functionality this package (libarchive13) provides?

$ apt-cache show libarchive13
Package: libarchive13
...
Description-en: Multi-format archive and compression library (shared library)
Archive formats supported are:
 .
    * tar (read and write, including GNU extensions)
...
    * zip (read only, with some limitations, uses zlib)
...
    * 7zip (read and write, with some limitations)

Moreover as further proof - if we try to remove this package, it will remove many core applications from GNOME (Nautilus, File Roller, Evince, GNOME Control Center and others):

$ sudo apt-get purge libarchive13 --simulate
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  evince* file-roller* fwupd* gir1.2-rb-3.0* gir1.2-totem-1.0* 
  gir1.2-totemplparser-1.0* gnome-control-center* gnome-software*
  gnome-software-plugin-snap* grilo-plugins-0.3-base* gvfs-backends* 
  libappstream-glib8* libarchive13* libevdocument3-4* libevview3-3*
  libgnome-autoar-0-0* libgrilo-0.3-0* libgxps2* librhythmbox-core10* 
  libtotem-plparser18* libtotem0* nautilus* rhythmbox*
  rhythmbox-plugin-alternative-toolbar* rhythmbox-plugins* totem* 
  totem-plugins* ubuntu-desktop* ubuntu-software*
0 upgraded, 0 newly installed, 29 to remove and 53 not upgraded.

So the answer is: Nautilus 3.26 in Ubuntu 18.04 LTS does not use p7zip, it uses functions from libarchive13 package.