Ok lets start with boot fixes.
These little things are required to run games on the PS3 system, the full form is encrypted boots these are encrypted if i am not mistaken with the firmware required to run the game itself that is either 3.55 or above, newer games will require higher firmware to run in order to play the game, with that being said these eboots are needed to be signed for lower firmware aka CFW 3.55 in order to play them on 3.55 firmware, hope this makes sense. And no you do not need to patch eboots in order to play on 4.21 as this is one of the latest firmwares minus 4.30, 4.25, and 4.31
This is simply in its simplest form Retail and Debug coming from the 360 scene you should know their capabilities and what the difference is now that i given this much. Although a retail machine is for user mass consumption and debug units are for development purposes only
Getting a PS3 lower than 3.55 is sometimes very difficult to obtain, try and get one that is 3.55 firmware and no CEX/DEX is not something to worry about.
I recommend you stick with 3.55 Kmeaw since you are relatively new to the scene, flashing is simple, you flash it like any other firmware but you need to be on 3.55 OFW in order to do so.
With Rogeros 4.21 CFW you wouldn't need eboot fixes like i mentioned earlier these fixes are required for lower firmware. But you man need to resign them to work with this firmware since most of the fixes are for 3.55 you will need to resign for 4.21 firmware.
Hope i helped if you need more information please let me know.