Appearance
Snapping, the grid and the view
Point snap
Always on, and not configurable. Any placement click within 12 screen pixels of an existing point lands on that point instead.
That is how two lines come to share a corner. Lines that snapped to the same point are joined at it; lines that only end near each other are two separate ends, and the profile they belong to will not close.
Because the tolerance is in screen pixels, it gets tighter in world terms as you zoom in. Zoom in when you need to place something near an existing point without catching it.
Grid snap
Off by default. The dotted-grid button in the top bar toggles it, and the setting is remembered.
With it on, placements and drags round to the visible grid. The grid step follows the zoom, stepping through 1, 2, 5 and their multiples so the cells stay a comfortable size on screen. Zoom in for a finer step.
It is also in the context menu, as Grid snap: on/off.
Grid snap helps rough things in at round numbers. It is not a substitute for dimensions: what holds a part's size is a constraint, not where the click happened to land.
Framing the view
Fit F, the expand icon at the bottom of the tool strip, frames the drawn geometry with a margin. On an empty sketch it resets to the default zoom, centred.
This moves the viewport. No geometry moves.
Also: scroll to zoom about the cursor, and pan with the middle button, with both buttons, or with any drag while a drawing tool is active.
Centring the drawing
Center, X and Y in the top bar move the geometry so its bounding box centre lands on the sketch origin. X and Y do one axis only.
Use this when a drawing has wandered away from the origin, which matters because the origin is where a face sketch's coordinates are measured from.
Anything anchored pulls back when the sketch re-solves. A Fixed point, or a dimension to the origin, will keep the drawing where it was. That is the constraints doing their job.
Fit against Center
Fit moves the camera. Center moves the drawing. If the geometry is fine and you just cannot see it, you want Fit.