Yes, the TCP/IP stack and Tor/I2P analogy for L2s is a common way people conceptualize it.
I see you are from the privacy community. I listen to Surveillance Report every so often. My journey actually went from Bitcoin -> Bitcoin privacy world -> general foss/privacy community -> Monero
Yes you can directly onramp into Monero with something like LocalMonero or Bisq
...Or swap from Bitcoin or another crypto using Trocador or Unstoppable Swap
Popular mobile wallets I recommend are: Monerujo, Cake Wallet, or Stack Wallet/Duo. Currently enjoying Stack/Duo
All open source and all good choices.