A sale and a licence are different sentences
Selling a shirt and permitting someone else to use your design are two different decisions people routinely write as one — and only one of them gives the design away.

Same crane, same moon, same size, same ink, same position on the chest — one on black, one on apricot. Two shirts, one drawing, both real at the same time.
That's the fact licensing is built on: a design isn't used up by being used. It can be in two places at once, which a shirt itself never can.
What does licensing actually mean here?
Permitting a specific use of a design, rather than selling a copy of it.
Selling a shirt transfers an object — one crane-and-moon tee leaves your hands and somebody else now has it. Licensing transfers nothing physical. It's permission: this design may be used, this way, by this person or business, while you still hold the design itself.
Why can the same design be in two places at once?
Because a design is information, and information doesn't run out when it's used.
The black tee and the apricot tee both carry the full drawing, undiminished by the other one existing. That's true of every design here, and it's the whole reason a single piece of artwork can be printed, published, and separately licensed without any of those uses interfering with each other.
What decision does licensing actually put in front of you?
Not "do I give this away" — it's which uses you're handing over and which you're keeping.
Those are two different sentences that get written as one all the time. "I licensed my design" can mean almost anything, from one specific use on one specific product to something much broader, and the difference between those is the entire content of the decision. Vague permission is the thing worth avoiding, not permission itself.
Which uses do people usually hand over, and which do they keep?
It varies, which is exactly why the question has to be asked explicitly rather than assumed.
Some arrangements hand over a single use on a single product and nothing else — the original stays yours everywhere else, including your own shop. Others are broader. Neither is the default; the point is that the scope is a decision, not a detail that sorts itself out.
Does licensing mean giving up ownership of the design?
No — ownership and permission are different things, and licensing only touches the second one.
You can grant a use without transferring the design itself. Giving away ownership outright is a different, much bigger decision — closer to the sale side of the distinction between what you made and what you call yourself than the licensing side of it — and it's worth knowing which one is actually on the table before agreeing to anything.
Is a one-person commission a form of licensing?
Sometimes, but usually not — most commissions are closer to a straightforward sale.
A design made for one person is typically just made and handed over, full stop. Licensing becomes the relevant question when somebody wants to use a design you made for something beyond what a simple sale covers — a second product, a different context, a use you hadn't already agreed to.
When does this stop being something to handle informally?
The moment real money or a real business is attached to the use.
A friend printing one shirt from your drawing for personal use is not where this gets complicated. A company wanting to put your design on a product line is. The distinction between selling a copy and permitting a use is the concept worth understanding yourself; the actual terms of anything with money on it are worth having somebody qualified look at.
How do I get one made?
Describe the design at JustOG — it's yours the moment it exists, before anyone permits or sells anything. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped.
Designs other people have published are in the shop.
The apricot tee and the black tee are both wearing the same drawing right now.