Building pieces of functional furniture that will be durable over time can be approached in a couple ways I think.
The most common examples we see in DIY mobile forums and YouTube fall under the first approach, which is to overbuild with heavier materials and more fasteners than a professional would use. If someone has the basic understanding of how and when to pre-drill holes, and they make some rectangles for a bed frame by butt-joining 2x4's with 3" deck screws, then links it together with some more 2x4's and sticks plywood on top, that is going to be a functional and durable platform for many years.
Maybe then it occurs to them to jigsaw out some holes, slap a piano hinge on it, and presto...a hinged door to access stuff under the platform.
Where furniture starts to exhibit higher craftsmanship is in joinery. I'm going to copy a definition and link a page that lists some different types for examples. Joinery is a part of
woodworking that involves joining together pieces of
timber or lumber, to produce more complex items. Some wood joints employ fasteners, bindings, or adhesives, while others use only wood elements. The characteristics of wooden joints - strength, flexibility, toughness, appearance, etc. - derive from the properties of the materials involved and the purpose of the joint. Therefore, different joinery techniques are used to meet differing requirements.
Woodworking joints
Joinery can let you accomplish more with less. I'm currently building an enclosure for both my large 'fridge and an window AC on top. It will need to be strong, but also lightweight and able to flex without failure. I'm not going overboard, but just basic dado and cross lap joints, when combined with Gorilla poly glue, let me use 2x2 and 1x2 to create the cabinetry with just a router and saw:
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Joinery can really be fascinating...I get a kick out of watching vids on old world stuff, Japanese and Korean joints most recently.
I would suggest you always use fir instead of pine...pine has some purposes, but unless it's for a visual effect, furniture is not one of them.
If you ask more specific questions, you'll get more specific answers