My mistake was I planted 3 grape vines and 2 apple trees in clay soil. I did make holes and mixed compost with clay. When I stuck a stick in by the apple tree because one needed support, it went in very easy so I assume this worked a bit? The holes were maybe 1.5-2 feet deep and maybe 1.5-2 feet in diameter.

I'm 19 and am living at home. The landscaper who did my parents yard did mainly the same thing with our two citrus trees as well.

If this wasn't a mistake, let me know! But, I feel like it was. Can I did in a circle around the tree where the clay is and add in compost or something to help fix the soil? Or what can I do to improve the soil... are they forever doomed? Thanks.

Also, the compost I use is 1.5 cu feet in a bag for $7.99 but it looks very high quality and has mushroom compost, chicken manure, bat guano, etc mixed in. I bought some cheaper compost last year and it was basically wood chips....... but maybe that's okay?

We have some trees down our street that drop their leaves twice a year and form clumps by the side of the road, I could surely use that... I'm just concerned about maybe they spray the trees or use chemical fertilizers but maybe thats fine.

Thanks! :)

All of that preceding was the how, now for the why.
Having a mature property with planted beds I can't dig organic matter into the lawn or beds. I can only top dress.
Clay is sticky and clumps together tightly. It also has very little organic matter or sand content.
Gypsum breaks the bond between the clay bits.
Lime corrects ph and helps with the nutrient transfer from soil to plant.
Green sand helps to break down the clay, has micro nutrients and minerals. It also adds the K part of the fertilizer mix. At some point I will replace this with coarse sand.
Composted horse manure is the P part of the fertilizer mix, as well as lots of soil friendly bacteria and organic matter.
Over the winter this mix dissolves and leeches down into the root zone and below so that when Spring arrives it is ready to work.
I add the Alfalfa mix for it's high nitrogen content, the N part. Most importantly it stimulates bacteria/fungus which worms like to eat. The more worms present, the more tunneling, the more worm castings (perhaps natures best fertilizer), and more break down of the clay soil.
I saw a huge difference this year in the way the grass has flourished and the topsoil seems to have been extended as well as the root zone, from the test cuts I made.

Obviously if you dig more organic compost into the clay it will help even more. My strategy is working even though I can't really dig compost into a mature lawn and planted bedding.

Sounds like you're doing it right to me but I wouldn't use those leaves from outside your own garden~ you never know what you'd be bringing into the mix. If you want to continue improving the soil, use organic fertilizers SPARINGLY, such as fish emulsion, bloodmeal, etc, and just continue adding organic material to the soil like it sounds you're already doing.

N-P-K Stands for the 3 main nutrients (Macronutrients) all plants need in large quantities. By 'large' quantities, I just mean larger than is found naturally in native, unamended soils. You'll always see 3 numbers on bags & bottles of soil amendments & fertilizers which stand for the ratio of each nutrient.
N=nitrogen for green leafy growth, too much can make plant prone to aphid/ other insect infestation
P=phosphorus for flowering & fruit, too much can interfere with other nutrients being absorbed
K= potassium for strong plants & disease resistance, too much can interfere with other nutrients being absorbed (namely calcium &magnesium)

There are other nutrients plants need as well(calcium, magnesium & sulpher) as well but usually the soil already has plenty.

Since you're in So Cal, You might want to get Sunset Western Garden Book. It's a great resource for every gardener in our region (Im in Northern Cal). You could probably check one out at the local library first. You could also find an older addition that would be just as good as a newer one IMO and a lot cheaper!
Have fun in the garden and don't worry if you have failures~ every gardener makes mistakes!