As a sibling comment already noted, using "--root" doesn't always work and a q&a mentions the problems:

https://askubuntu.com/questions/28619/how-do-i-install-an-ap...

https://serverfault.com/questions/23734/is-there-any-way-to-...

For example, rust's cargo can compile and install applications into `~/.local`, but it's pain to keep them up to date, so I prefer to use same tools from my OS (Fedora) distribution repo instead.

edit: should have written "where does dpkg put the files of the package ?" or "where does that put the Deb's files ?", sorry.

       --root=dir
              Set the root directory to directory, which sets the
              installation directory to «dir» and the administrative
              directory to «dir/usr/local/var/lib/dpkg».

For instance I always do `python3 -m pip install --user wormhole` or `pip install --user wormhole` instead of sudoing my way to permissions hell. Python apps get installed in $HOME/.local/bin, so no cruft for other users.

But stuff installed with deb packages often have hard coded pathways and assumptions of where and how their files are run.

So I think it's fair to ask in reply to someone implying that `dpkg --root=$HOME -i mypackage.deb` is as clean/same as a flatpack behaviour "It installs only in my profile, so another user on the same machine doesn't have access. You can't do that with a .deb!"... It's fair to ask "where do those files in the deb package go ?" because if it works as implied (no pollution of the OS) I am certainly going to start testing installing things that way.