Inside the object, the mesh.

Press Tab and a cube becomes vertices, edges and faces. Select them in three modes, see through with X-ray, pick whole edge loops, then model: Extrude, Inset, Loop Cut and Bevel. Clean a broken mesh and finish with a stool. The keys are the real ones of Blender.

STAGE

MMB orbit · Shift MMB pan · Wheel zoom

KEY IDEAS

Points, lines, surfaces

Object Mode moves whole objects. Edit Mode changes the mesh inside: every modelling tool adds, moves or removes vertices, edges and faces.

Tab: Switches between Object Mode and Edit Mode for the active object.
1 · 2 · 3: Vertex, Edge and Face select modes. The same selection, seen as points, lines or surfaces.
X-ray: Alt Z. You see through the mesh and a box also selects what is behind.
Edge loop: A line of edges that goes around the mesh. Alt + click selects it; Shift Alt + click adds one.
E · I: Extrude pulls new faces out along the normal. Inset makes a border of faces inside a face.
Ctrl R · Ctrl B: Loop Cut adds edge loops across the mesh. Bevel rounds edges with Width and Segments.
Clean mesh: No duplicated or loose vertices, no holes, no faces inside, no vertices in the middle of an edge.
Delete · Dissolve: Delete removes geometry and leaves a hole. Dissolve removes it and joins what is around.

IN THE REAL BLENDER

More options, same ideas

Everything here works the same in Blender, with the mouse over the 3D Viewport in Edit Mode. Blender has more options for each tool: here are the ones to look for first.

The select mode buttons are in the header, next to Edit Mode. With Emulate Numpad on, the number row changes the view instead: use the buttons.
Toggle X-Ray is the button with two squares at the top right of the viewport. Its opacity is set in the Shading popover.
After each tool, the Adjust Last Operation panel (bottom left, or F9) changes its values: Width and Segments of a bevel, Number of Cuts of a loop cut, Thickness of an inset.
E also works on vertices and edges, and I I insets each face on its own (Individual). This lab extrudes faces only.
Mesh › Clean Up has Merge by Distance, Delete Loose and more. The Statistics overlay counts vertices, edges and faces.
A bevel of a whole object is also a modifier (Bevel in the modifier stack) that keeps the original mesh editable.