So here's my understanding of how this stuff works... I'm throwing it out there hoping people can correct me if I'm wrong.
The basic idea of an antibody is that it prevents a virus from binding to a cell and thereby turning that cell into a manufacturing site for more virus.
So that's what it is. The more antibodies you have in your system, the more they can stop the virus from attaching, and thus they slow down the replication rate of the virus in your system.
Your immune system is designed to produce antibodies once it gets the "recipe" after encountering the virus.
- The idea of the mRNA vaccine was to use your own cells to produce the spike protein of SARS-nCOV-2, so that your immune system suddenly sees that protein as a threat and develops antibodies targeted at that protein.
- For the vaccine, your body will produce a boatload of antibodies once you get the vaccine. But eventually the protein isn't around any more, the antibodies degrade, and start to wane. BUT, your B cells (the antibody manufacturing sites) now have the recipe for that antibody, and have memory of that protein, so they can jump into action MUCH faster after having the vaccine than before.
- For natural infection, your body will do the same thing. It will produce a boatload of antibodies to neutralize the threat. The downside is that instead of your body being filled with [harmless] spike protein because of the vaccine, your body is full of active virus that is anything from harmless. But the same concept applies. Your body may see a drop in antibodies over time, but as long as your B cells have the "recipe", they can make more rapidly.
- Because your antibodies wane in absence of a threat, you are still susceptible to new infection [via Delta or Omicron]. But the idea is that once those B cells have memory to create antibodies, they can create them in enormous quantities.
Infection by the virus is a race. The virus is trying to replicate as much of itself as possible as fast as possible. Your body may not detect the virus until after it gains a foothold, and then it has to play catch-up. The vaccine or natural infection means that you've probably got antibodies swimming around in your system which give you a head-start in that race even though your B cells haven't started producing new antibodies.
If you've never had COVID or a vaccine, then monoclonal antibodies are an artificial way of upping your head start while your immune system tries to catch up and produce enough on its own. IMHO if it's a race, the question is who you're racing against. Omicron is Usain Bolt. Fast as %^&$ but not very violent. Delta is Mike Singletary. Fast, sure, but not by modern standards, but violent as hell. The original strains were Ndamukong Suh. Slow (by football standards, not by human standards), and will flatten you to a pancake and crush your lungs. Your B cells can go 100 mph (faster than any of them), but don't accelerate as fast as any of them either.
If you have some level of immunity, your body can produce enough antibodies quickly enough to beat any of them in a race, but they might roll right past your head start and make it unpleasant for a little while.
It's possible that because monoclonal antibodies are limited in numbers, maybe the head start it gives you over Omicron isn't enough to make that big of a difference in outcomes. I don't know.