In general, they get grants of cash from the US which they are required to use to “purchase” from US suppliers more or less any weapons (with few export restrictions). We’re giving them weapons but they still get to pick out what they think they need. This is a pretty good overview which seems like it’s mainly missing:

• The fact that congress authorizes aid, and then the White House is generally responsible for actually sending it. That’s important in cases like the most recent aid package congress passed, which Biden is at least partially simply deciding not to provide, which he is more or less able to do (the “more or less” is complicated and I don’t really understand it).
• A detailed breakdown of what shipments got “paused” and what aid has actually been delivered since then. Presumably, the White House is able to keep the details of this information secret. Currently, Netanyahu is claiming that they’re cutting off a lot of shipments they should be giving, and the White House is claiming that that’s wrong and they’ve been delivering aid as normal (and as far as I know not saying how much that is); it would be nice to know the detail of what’s being sent and who is lying (although I have a theory).