How does home internet with multiple wireless users affect internet speeds?

Hi,
I'm using a home DSL internet connection (from a local ISP through ATT) with several neighbors sharing the connection and connecting wirelessly.

The speed of the internet connection is rated at "up to 20 mbps" and generally when doing a speed test, I get around 15mbps speeds (from both wireless and ethernet connected mac computers). Occasionally the speed will drop to low speeds (around 2 mbps or less) for a period of time, and then come back to 15 mbps. Not sure yet whether this drop corresponds with our neighbors streaming/downloading/etc. wirelessly from their devices.

How can I determine whether the low internet speeds are caused by the internet connection itself, or by other users streaming or downloading?

In a scenario like this, does the modem/router prioritize one user/activity over another?

What other factors affect internet (& wireless network) speed  & reliability?

My wireless router supports setting up another wireless network (5G). Would using two separate networks on the wireless network (one for me, one for the neighbors) allow any benefit?

I'm trying to determine whether it would make sense to reduce the number of users on our internet (by having the neighbors get their own internet connection) or, alternately, to buy a more expensive service which promises higher internet speeds.

Any insight into this matter?

Thanks

Tom