First congratulations on a new engine.
I have installed tectonic from AUR and it works. 
I already have TeXLive 2020 installed on my system and it is updated every day.
So, why does tectonic download the same packages and fonts that I already have?
This is wasteful of time and bandwidth, So, kindly suggest a workaround or fashion one.
Thanks.
If Tectonic works for you, then you don’t need TeX Live anymore so you can remove it if you are bothered by the bandwidth usage.
Thanks for the quick reply @PHPirates.
May I then ask whether packages once downloaded by tectonic will not be downloaded again.
I am compiling a book manuscript that is package-heavy and I do not want the complete list of packages downloaded for each compile.
Finally, if repeated downloads of the same package do not happen, where does tectonic stash the packages it has already downloaded at some point in the past?
Of course it does cache those files, cache location for your OS can be found here: https://github.com/tectonic-typesetting/tectonic/issues/159
I will check whether that is also in the docs somewhere.
@PHPirates I don’t think that this is true. I love tectonic but it can’t replace TeXLive. There are some situations where you can’t replace pdftex with xetex (and so with Tectonic). And because tectonic do not allows -shell-escape we can’t use it with some packages, like minted, for example. And there are plenty more reasons to keep your LaTeX distro and to want to share it with tectonic.
@Kpym Of course it would be a nice optional feature. You are welcome to implement it in Tectonic to allow reusing package sources from elsewhere.
Note that this needs to take many situations into account, for example when Tectonic needs a package which is not present in TeX Live, it needs to interface with tlmgr to install the right packages, in order to keep the auto-download functionality present in Tectonic.