What are these capped, metal pipes in our yard?

It's impossible to tell as these are just generic threaded steel pipe. If this were mine and I wanted to know, I'd dig down and see what's there. That will likely answer your question.

If no underground utilities can identify them then it is very likely they were installed without a permit and so no service has record of their location or for what use. Do as jwh20 suggests and dig down. Their placement suggests they are auxillary water taps.

I had similar pipes all around my 50's era home. It turned out they were taps into water service (between meter and house) placing locations for spigots on all sides of house where there was originally just one located on the wall.

You could get lucky checking if these unknown pipes connect to known water or gas system using the continuity function on a multimeter.

If no continuity, then no useful information (as there could be several reasons for this). The 9 volt battery in the meter should be safe.

You will need to attach a long wire to the pipe, then take other end of wire to near water/gas pipes/faucets/spigot and use meter.

For a long wire you could use a normal extension cord.

You get an assistant to hold the earth pin on an extension cord on the pipe while you play with the other end.

Can also check if the two pipes are connected to each other this way.

If this is not clear, ask questions and helpful people will reply.

That end cap looks very similar to this.

1-1/4" Galvanized Malleable Iron Pipe End Cap Threaded Ends, 300 Liquid & Gas psi, 150 Steam psi https://www.mscdirect.com/product/details/36990166

This sort of cap is designed to resist high pressures. Whatever is in there, you don't want to risk it coming out in case you can't get the cap back on again.

As others have said, it's impossible to tell without following the line, so be careful. I've seen unused sprinklers capped like this, and it could also be capped electrical lines for yard lights.

they may not belong to official utilities and i suspect they do not. They may be places where electrical lines were ran from the house to support lighting. Perhaps they were never actually carried forward to completion of actually installing the lights but had the idea that perhaps someday and in preperation burried the pipe either with or without wiring inside ready to go. You can pick that kind of pipe and endcap up at any hardware store and all you would have to do is run a wire retriever down the pipe, they come on spools of different lengths, and attach the wires and then pull the tape back with the connected wires if there isnt wires already there. scope cameras are pretty cheap these days as well 12$ for one that hooks to your phone . make sure the power is off maybe before putting anything down there though if your worried about getting shocked but I dont see how that would happen.

https://www.google.com/search?q=fiberoptic+camera&source=lmns&tbm=shop&rlz=1C1GCEB_enUS932US933&hl=en&sa=X&ved=2ahUKEwjS1vyu9v3tAhUGlJ4KHU6QD_wQ_AUoAXoECAEQAQ#spd=13279337161738709542

https://www.amazon.com/Yaetek-Electrician-Wires-Cable-Puller/dp/B06VSPGJDN