Go to the Affinity Photo arrow tool in the tools panel and select that and you will the control bar change to include all of the features and settings for that shape. Change the color of the fill, set the color for stroke as well as the stroke width and dash etc settings and then change the settings such as thickness etc.
Once you have created a shape by dragging the cursor across the image while holding down the mouse then you will be able to modify the settings interactively (little red dots about the shape) by dragging them up or down or left or right. You can change the settings before you create a new vector (and they are all vectors, there is no pixel mode in Affinity) but if you create one with a shape already selected, it seems to re-set itself back to the default for the new design. You can set the thickness, type of ends (either end), length, inner offset (for right and left).
You can also fill it with any color so it can be filled with gradients and patterns via the gradient. The stroke and the fill can be set. You can also paste inside the vector design via the edit menu so if have select a shape then use the paste inside and you will then be able to create all kinds of unique fills.
The artwork itself can be expanded via the 'convert to curves' command which can be found in the control bar as well as just right clicking the design and then the nodes of your artwork can be modified so you can create even more unique artwork designs.
You can also add layer styles to the design via the layers panel and the fx for the current selected entry. You can then set the design with a drop shadow, inner glow, 3D, bevels etc. These can be altered at any point, they are live.
You can also rasterize this design via the layer menu and once rasterized you can use the artwork as a pattern layer or just use it with effects etc
You can go to the gear in the control panel and clicking that you will see all the saved variant designs of your shape. Serif have supplied a selection of starters. If you create an unique shape (and you haven't expanded it via the convert curve) you can go to the settings and add your own design to that gallery via the right side menu and create. You can then re-access those any time when design a new similar shape. The features such as the ends and other interactive settings are stored in your preset.
You can select a variety of ends. The ends are not so expansive as some applications but you can change the left and right very quickly via the drop down in the control panel and you have a selection of barbed, diamond, circular etc to choose from. Once you click the convert to curve, you can no longer change the setting via the control bar but as soon as your design becomes a curve, you can also modify the ends using the node feature of the application and therefore can add more points to it or remove or reshape the design using the node, You can also break it apart and much more. The ends can also then be overridden by just create a new design and placing it on top of the original and then you can either group (if the design is a complex design such as an animal or house etc) or if it is a basic but complex shape,m you can select both shapes and then go to the layer menu and geometry and add command and that will then combine the two and you can also extend to subtracting from the end as well. Or you could just select the nodes and move them to create an unique end design in seconds. Sadly, you cannot add any end designs to the end presets which is fixed by the application