>What legal grounds do they have for that evidence?
"FISA works both ways". It also allows for surveillance of those in contact with the target. If there is a FISA warrant on person 'A' the warrant applies to anyone person 'A' communicates with, for example persons 'B' and 'C'. This rule applies to several 'hops' or multiple levels of associates in communication with 'A, B, and C'. It means that if 'C' communicates with 'D, E, and F, then 'D,E,and F' are covered by the FISA warrant.
If POTUS spoke to Mueller and Mueller spoke to Weissman and Weissman spoke to Strzok then "We have it all", legally, using their own warrants against them.