Plywood is often stronger than solid wood, but most importantly it doesn’t bend when drying or in moist environments if it’s properly cross-laminated. However, cheap IKEA furniture is often laminated fibre, which is basically compressed sawdust. It’s an entirely different material.
Not just while the wood is drying, wood 'movement' is something woodworkers have to take into consideration in almost every project. Everyone lives in an environment with humidity (some more than others) and when seasons change solid wood will expand and contract. Plywood is very stable with respect to humidity.
That's a great idea, I'll definitely be doing that in the future!
I've added L-brackets to key spots on Ikea furniture before, and it often increases how sturdy it feels by an order of magnitude.
Just the other day I fixed a 5 year old Billy bookshelf (that I broke while moving it, while partly loaded with books) by securing the cardboard back with screws instead of finishing nails and adding a couple metal brackets to the bottom. Cosmetically, despite being (ab)used by my kid, it still looks fine, and now feels even more sturdy than before. Wish I had thought of adding some glue (or maybe No More Nails) to secure the back even more.
I only started doing it because I have a bunch of the Regissor line. It actually has a screwless assembly process, only being held by friction joints. Over time and after being moved a lot, the coffee table started to loosen. So I took it apart and reassembled it with wood glue in the joints, and now it’s as solid as a rock. I mainly buy the “higher-end” IKEA stuff with real wood or at least stronger particle or plywood or whatever it is except for the Kallax line, so that helps as well.
I just like IKEA so much because they have well designed stuff for good prices, even their more expensive things relative to their cheaper stuff. And with mild care and tweaks during assembly, it can be quite solid. I wish I had thought of the wood glue sooner as well because I’m too lazy to redo my bookshelves.
No, it depends on the application. There's a reason it's still called "solid wood".
If you look, most plywoods are "whitewood" or whatever is cheapest (most softwoods are incredibly strong as the term is just a synonym for coniferous). If you compare apples-to-apples -- Baltic Birch (or similar products with only birch layers) to solid birch...
The plywood doesn't have any noticeable wood movement;
The plywood can have fasteners and glue in any face (unlike solid where endgrain must be joined);
The solid wood usually has much better stiffness;
The solid wood is limited to shape and size by nature of trees; plywood can be found in larger sizes.
Good wooden chairs are impossible to find commercially now. The wood from the seat needs to be very wet and a wood that doesn't split easily, and the wood for the legs needs to be riven and dryer than what its final environment will be (so as close to 0% moisture as possible, at least <10%).
This is so that the legs expand a bit into the seat, which will continue to shrink, and make an incredibly tight fit. But it's not conducive at all to mass-production, which is why you often see them nailed, bracketed, bolted, etc, all sorts of dirty tricks.
Woodworking is a rabbit-hole you might find compelling or infuriating. You can't unlearn the knowledge when you do.