That Railjack 3.0 upgrade has replaced Avionics for Railjacks, turning it into a more traditional mod system. This works very similarly to the mod system for Warframes, so shouldn’t contain too many surprises.
All players now have a Plexus, a piece of equipment they are automatically given that comes into all Railjack missions with you. Avionics have become three different types of mods, and you can slot them into the appropriate part of the Plexus.
To access the Plexus, you can visit the Dry Dock, you Orbiter, or the Dry Dock in any Relay.
To place mods, open the Plexus and you will see the below screen.
It will contain three tabs. Your Integrated Mods that are split between the Aura Mod and Personal Mods, the Tactical Mods, and the Battle Mods. Much like Warframes, your Railjack will have a maximum capacity, and you can place the mods you want as long as it remains below that capacity.
Upgrading Mods is identical to the Warframe Mod system, and all Dirac has been converted into Endo for upgrading purposes. All spent Dirac has been refunded, and all other Dirac will be converted to Endo at a rate of 1 to 1.
Once again, you will only be able to upgrade mods to your maximum capacity, if you upgrade a mod beyond that point you will not be able to use it.
Mods can be earned in Railjack missions, and will often drop from slain enemies and can then be picked up by flying the ship through them, so keep a close eye out for drops.