So basically, the Big Turrets would occupy 2 slots that are next to each other (turning it into a single big slot). You would need to build 2 "normal" turrets next to each other and then have the required Big Turret technology (only available to get, if you have technologies of the 2 "normal" turrets). There will be a button between 2 turrets that you could press to combine them and get a Big One (maybe for a bit of resources, not much; maybe for free, considering the cost of "normal" turrets)
How it would work: if we take for an example a Big Turret made by combining laser and minigun turrets we would need to follow 4 steps to get it: 1. get laser and minigun turrets technologies; 2. get the Big Turret technology; 3. build a laser and a minigun next to each other; 4. combine them to get a Big Turret. After all that you will get a Ray Turret that would fire a ray that gets stronger the longer it attacks enemies, but overheats if it attacks for a long time.
Some extra stuff:
For swarnids platform: the 2 "normal" turrets must be close enough to each other for the combine button to appear, if we are talking about the fact that they can relocate their turrets. And after combining there will be a big slot with the Big Turret that you can also move around.
If you combine turrets that already have upgrades, then those upgrades would be fully refunded.
If you want to destroy a Big Turret it would cost you 2 iron. And after destruction there would be 2 "normal" turret slots in place of the big one.
The Big Turrets could get more stats from upgrades than "normal" turrets. For example: while normal turrets get a 35% bonus to attack speed from an upgrade, the Big Turrets would get a 50% bonus instead.
In order to unlock the ability to find Big Turrets technologies in a run, you would have to purchase an upgrade from the upgrade tree in the main menu.
Big Turrets inherit turret types from normal turrets.