Can someone ELI5 what amp 747 means

It's based off of this chart from the wiki. A 747 amp would be the Klamora Prism, Phahd Scaffold, and Certus Brace. Amp parts 1-4 are from Onkko in Cetus, while 5-7 are from Little Duck in Fortuna.

Amps are made of 3 component parts, Prism - Scaffold - Brace.
Also, the set parts available on vendors were numbered (for convenience sake) based on the order you unlock them on Cetus (1 to 4) and then the ones on Fortuna (5 to 7).

So, when people say 747, it means [7th Prism] - [4th Scaffold] - [7th brace].

Amp parts are divided into tiers: 1-4 from Onkko in Cetus, 5-7 from Little Duck in Fortuna.

First number is the Prism, second number is the Scaffold, third number is the Brace

In this case, 747 would be: Klamora Prism, Phahd Scaffold, Certus Brace

Amps are crafted part by part, then put together by the quills

amp parts are unlocked 3 by 3 each rank at the quills. The parts of the first rank are the "1" parts, the ones of the second are the "2" parts, etc...

there's 4 tier of parts from the quills, so it's 1 to 4

in fortuna, the hidden syndicate Vox Solaris propose 3 more set of parts, so they are named 5 to 7

A 747 amp means it has the prism 7, the support 4 and the brace 7.

You make amps using a prism, a scaffold and a brace. Mushroom hat man Onkko sells 4 different part sets of prism, scaffold and brace. Backroom robot lady Little Duck sells 3 different part sets.

In the order they are shown in Onkko and Little Duck's shops, the sets are numbered 1 to 7. So for example if you want prism 4, scaffold 3 and brace 7, then you would call it a 437 amp. A 747 amp is an amp made with prism 7, scaffold 4 and brace 7.

Custom-built Amp parts of tiers 7 for the first segment (main fire), 4 for the second (alternate fire), and 7 for the third (stat boost/tradeoff).

Since none of them given you ELI5 version of what it means:

7: Short Beam Cannon +

4: Glaive Throwing +

7: High Critical Chance

in your AMP that all those terms also exist for weapons for you to relate.