What I am really curious about is, well mainly what allowed Phillip II to come into Greece, and conquer what was almost a unified force, prior to the collapse of the Delian League. (Also apologies if my question is poorly informed, I am recently interested in ancient Greek history and this is what I have kind f pieced together)
There is a significant amount of time between the dissolution of the Delian League (404 BC) and the conquest of Greece by Philip II (338 BC). In fact, there was time enough for the Athenians to start a Second Delian League (probably in 378 BC) and for that League to fall too (after the Social War of 357-355 BC).
However, Athenian decisions certainly did play into Philip II's hand. Firstly, from 368 BC onward, the Athenians waged a costly war to recover the strategic city of Amphipolis in Thrace, which they had lost in 424 BC. They failed to take it, but antagonised the Macedonians in the process, and Philip eventually did conquer the town (and its mines and vast resouces of timber).
Secondly, the Athenian attempt to recover their power in the Aeagen was initially successful, but their continued campaigning against Amphipolis made their new allies feel used for the sake of Athens' interests, leading to the Social War I just mentioned. This war was extremely costly for Athens, and decisively ruined its chances of uniting the Aegean in a new Athenian empire.
Thirdly, Athens spent a great deal of resources trying to set up a pro-Athenian government on the island of Euboia, which brought it into conflict with the Thebans through most of the 340s BC. The Thebans themselves were already extremely weakened by the ongoing Third Sacred War (356-346 BC) against Phokis, in which Philip II became more and more involved after he conquered Thessaly.
All of these wars and alliances are very complex, but the main point is that only major states like Thebes and Athens had the strength to form an alliance capable of stopping Philip, and thanks to their constant warring no one trusted them to have the interests of Greece at heart. Meanwhile, Philip encroached on Central Greek affairs more and more, while also expanding into the Hellespont, which was a direct threat to Athenian interests. In the war that followed, Athens' only major ally was actually Thebes, but their joined army was defeated at Chaironeia in 338 BC, and Philip made the mainland Greek subject to Macedon.