Easiest option is to put plywood on top of the joists (the wood I-beams). Unless they're spaced very far apart, they can easily handle the load. If it was a "normal" floor, you'd use inch-and-a-quarter plywood. If it's just going to be a storage area, you could use a little thinner plywood as long as the load wasn't extremely heavy.

If you want something that's under the joists, the easiest are various tray systems that are suspended from the joists. Some of them are fixed platforms, others are suspended by ropes so you can get stuff out more easily. If you google "garage overhead storage" you'll find plenty of options.

the loft would hang from the joists below the garage ceiling

i did see the packaged products that provide metal framing and hangers, however i was thinking of framing with lumber and the hangers are what I was unsure about

I did see that some people use threaded rods for hangers which i suppose i could position on both sides of the joist and bolt them through a steel plate across the top of the joist

They're bound to have an opinion on the matter.

Other than that, I got nuthin'.

Then a 2 x 4" cleat, lag screwed or toggle bolted to the 2x3's would be sufficient.

Were it me, I'd probably do both- toggle bolts on the ends of the cleat, lag screws along the length- kind of a 'belt AND suspenders' measure.

If you're worried about screwing into the I-beams, you could use a length of threaded rod, and make a 'sandwich' of sorts- cleat along the ceiling, another 2 x 4" in the attic, with threaded rod & washers/nuts connecting them. That would be a bit of a pain, but it would insure that you don't mess with the engineered trusses.