A symbol is a vector already created that we can incorporate into our own designs.
To incorporate a symbol we have to activate its panel through the top menu "Object> Symbols ...". So we choose a category of symbols, select one and drag it onto the page.
What is created is an instance of the symbol, that is, a clone. We can modify it in size, position, rotation, color, etc., but we cannot edit its nodes. If we want to do it, we have to select the symbol on the folio and then Edit> Clone> Disconnect clone. We convert it to a path and we can now edit it freely.