Is it OK to use human waste or spent cat litter directly into the soil where you grow fruits and vegetables?
Not unless you particularly fancy a dose of e-coli, worms, parasites and a host of other enteric nasties you could be infected with from using it.
Generally speaking, manures from herbivores are okay, but even those must be composted prior to use, so composted animal manures are fine - but only if you're not growing root crops such as carrots or parsnips, which will fork if they're in manure enriched soil. And a word about 'fertiliser' - there are two approaches used to feeding plants in the ground, often used in combination. When you apply humus rich, composted materials (such as animal manure or leaf mould or spent mushroom compost or garden compost, anything like that) you're actually improving the soil, and the bio diversity within it, and its general mineral content, depending what you use, thus enabling your plants to access the nutrients they need themselves. You can also feed plants directly by using proprietary 'chemical' feeds such as Growmore or fish blood and bone or any of the other hundreds of proprietary fertilisers. You may, for instance, choose to use tomato food on tomatoes, even if you're growing in the ground, and the ground has been enriched with humus materials beforehand, because that tomato food supplies nutrients tomatoes particularly need.
In some parts of the world, human excrement is used as fertiliser, but not without processing first, usually using high temperatures to kill off the nasties.
If you are going to do what you ask, use human and pet wastes to fertilize your 'fruits and vegetables', there are a couple of things to keep in mind:
1. Processing waste material
Human wastes can and have been used to fertilize crops. The city of Milwaukee sells Milorganite to farmers, which is just processed human waste from the city.
A good minimum for home preparation of human solid wastes is layered with a substance like sawdust or ash, left 2 full years sealed off, then composted with other organic matter for another year.
2. Application types
To apply organic fertilizer to fruit trees or bushes, you can spread out the fully composted mix in a radius around, but removed from the trunk. Lightly incorporate into the soil, then cover dry organic material.
Incorporating your fertilizer into soil for vegetable crops (like roots, but even lettuces, less so vines) is where you are going to want to be careful. Human wastes contain harmful microlife, so you will want to be very confident in your process (maybe even going a bit too far). The risks are high so it really is recommended not to use this home mixture for direct application on annual vegetable crops.
It depends on what you mean by human waste.
If solids, then these need to be composted first. A hot composting system can render these into compost in 18 days and can kill off all pathogenic microbes.
If not, then you need to leave it for longer, up to 2 years depending on the composting method.
If liquid, then it needs to be diluted before being delivered to the soil otherwise you might burn the roots. Urine has a NPK ratio of approximately 10:1:4, and needs to be supplemented with wood ash if you want to use these as a complete fertilizer.
Note that pathogenic e-coli can be taken up into the stems of plants so don't use untreated urine if you have a urinary tract infection.