How can I (hard) landscape my garden for no money?

If you have transport, there's plenty of free used materials going. I've made a path from roof tiles!

Perhaps the raised beds could be repaired with wire mesh behind any cracks and stabilising plants? I've used violets and bell flowers to fix the soil on banks with their long roots.

Maybe think in terms of reusing as much as possible. The retaining walls could still have many good blocks that just need resetting with new cement.

Ask for broken paving stones, people are frequently happy to give them away rather than pay a fee at the tip. You might be lucky and they will deliver but other than that you'll have to arrange collection.
Stick an ad on you local neighbourhood site and facebook etc.

When we were making a patio we asked on gumtree for slabs and were inundated!
The problem was fetching them as they were heavy. Our wee trailer got a puncture etc. but I was amazed by the number of people who were upgrading their gardens by buying new monoblocks and were delighted to have us remove their old paving stones. We were offered bags of gravel too. The 2 x 2 ft stones we could manage but the 3 x 2 were enough to give you a hernia.

Old bricks are good too but our best investment, in pre-covid times, was hosting volunteers who worked tirelessly to help us.
www.helpx.net/index.asp
www.workaway.info/

Hang around your local DIY stores - staff routinely sling broken slabs into a corner somewhere before sweeping the lot into bins.
Check your local freebie website - surplus items from all sorts of projects are advertised on those sites.
Drink a lot of wine (perhaps cheaper to sneak around and rummage in re-cycling receptacles) - bottles pushed down into earth make a path, bit slippery though, better pushed into bank of earth to make a retaining wall.

Have a look on Freecycle or a local buy and sell Facebook page for scaffolders planks to build low beds. They're hard wearing and will last long enough to see you and me out! Ditto for unwanted paving slabs or used bricks that you can use to lay a herringbone path. The more random the bricks, the better it looks. I don't know whereabouts in the UK you are, but our local buy and sell Facebook has had several sheds to give away this summer, the caveat being that the person who had it, had to dismantle it.

Do you have funds to buy some materials but not to pay for labour? A few photos would be helpful to better appreciate the problem.