At level 20 you can begin to make nanoweapons. You must meet all the requirements to get your Gear Factory to level 20, but once it is upgraded, making these weapons is easy.
Once you make this happen, you should always be doing two things:
Producing nanoweapon parts (materials)
Using nanoweapon parts to make nanoweapons
Once you have opened up this feature, you should never not be doing these two things. They cost you nothing but a few clicks, and the benefits are enormous.
Once a five star hero has been evolved to level 211, an option will be unlocked on that hero that allows you to equip one of these nanoweapons.
Some weapons are good for everyone in a general sense, however many of these weapons have bonuses when equipped on a specific hero.
(It is important to remember that these nanoweapons are produced randomly)
Because of the random nature of nanoweapons, you are unlikely to get a specific one until you have made of a few others. Most often the weapon you will produce is a general one, the terrestrial radar seems to be the default, and likely you will produce one of these
Rarity
Normal Craft
Exclusive Craft
Common (Green)
63.9%
49%
Uncommon (Blue)
25%
30%
Rare (Purple)
10%
15%
Epic (Yellow)
1%
4.5%
Apex (Rare)
0.1%
1.5%
While we know the percentage of crafting rarities, I am unable to find documentation around how often one weapon will be produced against another. But my experience is that terrestrial radar is produced at least 75% of the time.