Take a look for the thread "patchmanager" on this forum.
It can be downloaded from OpenRepos, which is an unofficial application repository (aka "store" in the rest of the world), where most of the applications done by the community are pushed, to be faster than the official Jolla store and overcome some of its limitations.
There are graphical applications to use OpenRepos from the Jolla which are "StoreMan" and "WareHouse" (not sure this last one was updated, it broke some versions of Sailfish ago IIRC).
Then patches can be added from several sources and applied through the application PatchManager.

It depends on what you intend to do.
SSH is the basic interface to have a command line access to the phone from a PC through USB or the network (Wifi).
Locally, you can also get command line access using the terminal application (shown when you activate the developer mode in the settings).
If you don't like command lines, there are some things that would not be as easy to do when hacking the phone, as there are not graphical user interfaces for all things.
If you only intend to use the phone as a smartphone, you should not need command lines, as applications and already done patches can easily be installed graphically.
Lastly, if you want to develop apps for it, the Sailfish SDK is the tool to develop Qt/C++/QML apps from a computer and debug it. Other kind of apps can also be developped depending your prefered languages, like python, that can be done on device or also remotely (with or without the SDK).