I have a question for using off-camera flash for a Canon 6D. I have two flashes. One is a Canon 600EX-RT and the other is a Digitek. Can I use different brand flashes for off-camera firing? What type of trigger and receiver do I need?
Lots of different radio triggers work with lots of different brands of flash. Yes, you can mix'n'match flash brands (see: What should I look for in a wireless flash trigger for a home studio?), but you may have to give up some/all additional flash features aside from firing in sync. Basically, you just need triggers that can communicate properly between your camera and your flash. The simplest/cheapest triggers are ones that only transmit the "fire" signal. More expensive ones transmit things like eTTL, HSS, 2nd curtain, and remote group/power settings.
In the case of the 600EX-RT, if you want to use it to its built-in wireless radio capabilities with your 6D and the full RT feature set, you'll need a transmitter that "speaks" Canon's RT system, otherwise, you'll be limited to the functions other radio triggers allow. The Canon 600EX-RT or ST-E3-RT are "masters" in this system. And there are several 3rd party clones that can work as well. But these are going to probably be among the more expensive class of trigger.
Your Digitek flash is the unknown factor. If it is a manual-only flash, you can easily trigger it in the RT system with a 3rd-party RT-system receiver, such as a Yongnuo ST-E3-RX, or Phottix Laso, but it won't understand HSS/TTL/etc. commands, just firing, and must have its power set manually. If your flash is Canon eTTL/HSS/etc. compatible, it should work with those functions as well, but may not have some of the addition RT function capability, such as being able to be used as a remote shutter release for your camera, unless the trigger you use adds that function.
You can also use non-RT TTL-capable triggers if you want, but you're liable to lose access to some RT-specific features, like ID codes, Groups E and F, Gr(oup) mode, and using the flash as your shutter remote. And if you use manual-only triggers, you'll only have firing in sync. This all depends on the triggering system you choose, and how that triggering system communicates with your gear.