Can someone please describe what the exact monopoly position is that Google have and how they defend it? I take it it's some sort of ads marketplace where they

Can someone please describe what the exact monopoly position is that Google have and how they defend it? I take it it's some sort of ads marketplace where they charge a fee to connect buyers with sellers? But why are things like chrome and payments to companies for default search engine placement considered monopolistic? It is increasingly sounding like a witch-hunt given the list of accusations I've heard.