North Korea is very poor and cannot afford modern air defense systems. There is also an international arms embargo on North Korea which prevents North Korea from buying new weapons even if it had the money. North Korea has to improvise and does this by (sort of) keeping older weapons and aircraft in service. North Korea has a lot of elderly air defense artillery and SAM (Surface to Air Missile) systems and puts most of this round the capital and some key military installations. This may improve North Korean morale but any potential attackers, like South Korea or the United States plan to use guided missiles against these targets. Anti-aircraft guns and obsolete SAMs can do little against this sort of thing.
North Korea also has hundreds of nominally operational Russian aircraft. Only a few dozen MiG-29 and Su-27 aircraft, which entered service in the 1980s, are available. Most (about 600) of the North Korea jet fighters are 1950s designs. This includes nearly 500 MiG-15s and 17s plus about a hundred MiG-21s and 23s. North Korea manages to keep these aircraft operational by only letting pilots fly them one or two hours a month. That means pilot skills are as limited as the effectiveness of these elderly jet fighters.
While North Korea has been unable to afford new jet fighters, UAVs are another matter. North Korea has been able to obtain several hundred of these from China and Russia. Many of them are over 20 years old and many have a range of over 400 kilometers, but few can carry weapons. North Korea has sent some of these into South Korea on reconnaissance missions but the South Koreans learned how to spot them and shot them down using jet fighters, armed helicopters and ground fire. As UAV losses increased, North Korea halted the cross border flights. South Korea is prepared to do the same to any North Korea jet fighters or bombers. North Korea has about 80 Cold War era Il-28 Russian bombers. North Korea is the only country still operating these 21-ton two-engine bombers. The Il-28 was the first Russian jet bomber, over 6,000 were built in the 1950s, and most were retired in the 1960s because faster and more capable jet bombers became available. Russian exported the IL-28 to dozens of countries, including North Korea. The Il-28 has a crew of three and carries up to three tons of bombs. Top speed of the IL-28 is 900 kilometers an hour but the more economic cruising speed is 770 kilometers an hour. At that speed the IL-28 has a max range of about 2,000 kilometers. Most countries with IL-28s retired them in the 1970s and 80s. North Korea kept theirs because it was the only jet bomber they could get.
One elderly Russian military aircraft that remains very useful is the AN-2 biplane transport. North Korea has several hundred of these and they are to be used in wartime to sneak North Korean commandos into South Korea. An AN-2 can carry ten of these troops at slow (under 60 kilometers an hour) and low altitudes that modern radars cannot detect. South Korea modified some of its air defense radars to detect AN-2s and shoot them down before they can deliver their commandos. Those commandos that do reach South Korea are supposed to create as much havoc and confusion as possible. AN-2s can also carry two tons of cargo or bombs. The bombs would simply be armed and pushed out the cargo door by a second crew member. Normally the AN-2 is flown by a single pilot.
Over 18,000 AN-2s were built and production continues in some countries. For North Korea, the AN-2 is the only really effective military aircraft they have.