Isolation isn’t always obvious.
One way one of my exes would isolate me was by being rude to my friends whenever they came over. I had moved to a new city and was trying to make new friends outside of my boyfriend’s friend circle. I had two people literally never speak to me again after meeting him.
Another way he isolated me was by trash talking my friends after I spent time with them. I would spend less time with my friends just to avoid having to listen to him go on and on about what he didn’t like about them. He was overly critical of them which also felt overly critical of me because I chose to spend time with them. He implied I should find better friends but it would be impossible to find friends he approved of.
One of my other exes would flirt with my female friends in front of me. He would tell me he wanted an open relationship without boundaries in which we (he) would be allowed to date anyone he wanted including my friends. He would tell me how, shortly before we started dating, he wanted to sleep with some of my closest female friends. As time went on I had fewer and fewer female friends, was further isolated from my feminist circle, and eventually from feminism in general as he would start fights, tell me I was annoying when I pointed out sexism, and say that I was reverse sexist against men.
One thing that two of my exes did was embarrass me when we went to parties. I would go to parties to interact with newer friends and make new friends. They would act so ridiculous that I would leave parties far sooner than I wanted. There was also a pattern of them getting (or acting) so drunk that I would have to take care of them or leave parties early so I could get them home.
My ex also used to criticize my social skills which made me feel inept at making new friends. The worse he treated me the worse my self esteem would get and the less I would try to make new friends or spend time with the ones I did have out of anxiety of being rude or coming off as selfish or uncaring. I started to believe that no one could think I was a good friend and that the friends I did have probably secretly disliked me.