Multi-Sampler Not Showing up in Logic/Where do I Find the Multi-Sampler in Logic?

I think, you are looking in the wrong place. You need to select the sampler or any other instrument in the Instrument Slot of the Channel Strip. The library on the left side is just a collection of sounds, no matter of which Instrument they come from.

I see. It should look like this:

I would create a complete TimeMachine backup, then update Logic and your MacOS since both are out-dated .

On a un-related note, I'm going to keep using Sonoma because plugins aren't always compatible or are buggy with the newest MacOS and I don't want to risk going through the trouble to down-grade back to Sonoma if that happens. Is this something you experienced perhaps, or is it not a worry for you at all?

Of course, don't change a winning team. In may case updating to sequoia fixed some bugs with Logic. And I didn't run into compatibility issues on my system. But I always create a time machine backup right before I do an update.