What is wrong with this apple sapling?

Question

I have an Opal apple sapling that I've been growing since she was a seed: only for the joy of watching a plant grow; not with any intent that she become fruit-bearing.

She's spent her whole life in pots (changed when I feel she was outgrowing one) and I've always kept the pot inside a mesh tent for fear of critters. Her current pot is conical: 12" top diameter, 9" bottom diameter, 11" tall. She is currently 18" tall from the ground.

I felt that she was strong, growing and healthy for well over a year: thick growth of well-formed, deep green leaves. Over the past two months, though, she's been developing a worsening symptoms on her leaves: at first a few odd brownish dots that I brushed off as the "plant equivalent of birthmarks or natural imperfections". But then the leaves started growing with odd deformities: holes of varying sizes, and sometimes large percentages of the leaf outright missing.

The only apple-related potential match I could find on Gardening Stack Exchange was this one: Apple Tree Leaf Growth/Disease? Cedar-Apple Rust? -- but those pictures don't exactly match mine: that apple tree's leaves discoloration was yellow/orange and uniformly circular, whereas my leave's discoloration is brown (the color of a dried leaf), and has irregular boundaries. Also the other post didn't show any leaf deformities.

I've wondered whether a change in the sapling's watering was the onset: until about two months ago, I'd always watered her with the same filtered drinking water that we humans drink in our household. When we went on vacation two months ago, I set her up with a drip irrigation line (it looks like this product: https://www.rainbird.com/products/micro-bubblers). I don't know the specifics of our sprinkler water quality; it's certainly not the filtered-for-humans water that she grew up with, but it's kept several plants and trees in our yard alive for many years.

She lives in south Orange County, California: I believe we are "BSh (Hot semi-arid)" per the Koppen climate types. Earlier in my sapling's life I used to bring her outside in the daytime for sunshine, then bring her inside at night; over the past two months, I've left her outside all day long, partly because she's getting too big for me to move her frequently, partly because I felt (nothing more than a gut feeling) that she was mature enough to transition to a fully outdoors lifestyle.

That is my sapling's history, and my thoughts at relevant changes in her recent history. My plea to the gardening community is: can anyone please help identify what the problem is, and any cures for my poor apple sapling's ailment? Is this a disease, a harmful critter, or unsuitable weather?

I've scrutinized the sapling as best as I could, and as far as I can tell, there are no insects or other critters visible to the naked eye on the plant or in the soil.

Below are pictures of the various blemishes and deformities on the leaves, that I've described above:

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Update:

I thought this was perhaps noteworthy: as I mentioned above, my little plant was strong and healthy her first year of life; now interestingly, the leaves from that healthy phase of her life are completely untouched: they remain unblemished, a healthy deep green, and without the "munched on" appearance of the leaves in the pictures above. If critters were munching on these leaves, surely they would be indiscriminate, and the eating would occur everywhere, all up and down the plant, no?

Answer 1

The leaves eaten from one end to the other have been hosts for larvae probably Epiphyas postvittana (Light Brown Apple Moth)

They are gone now. But be prepared: They will return!

Life cycle projections for the areas of California where it has been found indicate that four to five generations are possible. (per year)

Only having one tree, manual removal of eggs is the best way to fight them:

Females deposit egg masses containing 20–50 eggs on the upper leaf surface or on fruit.

Small round holes and brown dry areas are often caused by snails and slugs. They are easy to identify by their slime trails. They hide in the soil during the day and are only active in the night or when the soil and leaves are wet. You can catch them in a trap e.g. a beer trap.

Answer 2

I'm unable to confirm @gyrfalcon's answer that the offenders are Epiphyas postvittana (Light Brown Apple Moth).

Broadly speaking, we do have moths in the area, but not to an extent where I notice them any more often than every few weeks or months or so -- i.e. they don't present themselves as a widely-recognized pest (locally). I haven't seen any moths recently in my daytime or nighttime inspections of the apple sapling either.

But I did do a thorough examination of my little plant, and found a few things:
First: I was given a reality check that the tent that houses my apple sapling realistically has a mesh with holes too big to prevent aphids getting through.
A thorough scrutiny revealed that indeed there were aphids on my innocent apple sapling! I've burned aggression over the past few days with a daily aphid massacre, carried out by squishing them between my fingers. I will probably move on to spraying my apple sapling with a soap solution, as seems to be frequently recommended.

Second, I moved my apple sapling out of the path of direct daytime sunlight.

Coincidence or correlation I do not know, but those two actions: manually killing aphids, and moving the apple sapling out of direct sunlight seem to have resulted in several days where new leaf growth has returned to being unblemished and not deformed.

I debated whether to post this as an answer or an update to the original post because it's not clear that I've actually found what the original problem ever was: I haven't found any articles showing either aphids or sun-damage causing any of the problems in the original post/pictures: "deformed" leaves with a "munched on" appearance or irregularly-bordered browning. Also, during nighttime and post-watering inspections, I never saw any snails/slugs.