This is what I think the problem is. I’m not complete sure on how to remove it.
I see someone put the proper dielectric union between the galvanized and copper on the cold. Can't tell on the hot with all that scale but the shape looks to be the remnants of a union. Wow!
That is definitely a DYI job.
Cut both copper pipes, and see if you then turn the rotted galvanized elbow off the galvanized straight. Use a little heat on the elbow if you need it. If it breaks off the galvanized straight, or once off the threads on the straight are really bad you have two choices.
The hard, cut back about 5 inches on the straight galvanized line and re thread....or the "easy" find where that straight ends and go ahead and take it off and that junction if you can. A couple of good pipe wrenches, some WD40 and some heat will do the trick if it's tough.
Then, wrench on a pex transition fitting to the galvanize pipe and solder two pex transition fittings on the copper pipes.
Then it looks like about 10 feet of pex, one pex "T" and about six crimp rings and you are ready to go. Hopefully that was the problem. If not.....would hate to see what is. Good luck.