Neutrino (BIP157) is the service that is syncing the block headers. That is a small part of data and also quite private. You can connect to the LSP bitcoin node or any other bitcoin node (even random) that is providing blocks through neutrino.

From there, the LND node, linked to these blocks, is downloading and syncing the LN graph (the map of all public nodes and channels) from your LN peers you are connected.

In Zeus case I don't know exactly how is syncing this LN graph, but in Blixt case I know that they setup a server node that is providing a quicker and compressed copy of the LN graph that you, the user, download faster when you open the app and you are ready to transact over LN faster with a cleaned up map of zombie channels. In the past was the issue that Blixt mobile nodes, because of low latency connection were downloading a wrong map with a lot of zombie channels and their payment were failing a lot because of that. Now is working really nice. In the future could be better protocols to getting faster this initial sync at opening the app.

People using these mobile nodes MUST remember that if you do not have a good internet connection this sync could fail a lot, so your wallet. so have patience at least when you open the app and also try to open it almost every day to have a quicker update and better routes.