Sketch with the Freehand tool
Press F and draw. While the button is down the stroke follows the pointer; when you release, the tool thins the trail at its Smoothing setting and fits a curve through what is left, so a stroke of 151 samples comes back as 12 editable nodes rather than hundreds of points. Turn Smoothing down to keep more detail, or up for fewer, rounder nodes.
Turn on Shape Recognition and a rough stroke that was meant to be a basic shape becomes the clean shape.
Place anchors with the Path tool
Press B. Click to place a corner, drag to pull a handle and bend the segment, and click the first anchor to close. The pen draws cubic segments by default and can switch to quadratic, arc and Catmull-Rom segments in its options; Alt+click an anchor to toggle it between corner and smooth. Every anchor and handle stays live for the Edit tool afterwards.
Shapes with live controls
Rectangle with a corner radius, Ellipse, Polygon with any number of sides, Star with arms and an inner radius, and the other shapes: cross, ring, spiral, gear, arrow, speech bubble, heart, lightning and cloud. Each keeps its parameters in the Geometry panel until you convert it to a path.
Edit what you drew
Press E and drag anchors and handles, double-click a segment to add a node, or open the Path Editor panel to retype a command. Combine shapes with the boolean operations, cut them apart with Divide, and convert text or shapes to paths when you need nodes. The SVG path editor page covers the whole toolset.
Export plain SVG
What you draw is ordinary SVG: rect, ellipse, polygon and path elements carrying the paint you gave them. Export it as SVG, or as PNG, JPEG, WebP or ICO from the Export panel, and it opens in any other tool.