Does anyone know where to find a list of what phone companies use EE's network?

I read something about some not actually using EE's full network speeds or something, so not sure if it's worth going with one of the other operators that are using the EE network if its not going to be at the same standard.
Looking to get the best sim only deal with lots or data as our home internet is shockingly slow and EE is the only network we can get a proper useable Internet connection.
Thanks in advance for any help!