Are you having a problem? The copper is probably a compression fitting.

It should just unscrew.

If you are adding a filter can I assume you are prepared to cut one or both pipes then re-do fitting(s) ? If this water supply is something that you can't have inoperable a day or two I advise being prepared to thread and/or solder before attempting to unscrew it.

Theoretically you should be able to hold galvanized fitting with wrench while unscrewing middle , brass nut. The copper pipe looks to have kinked and I suspect it was because small brass nut, large brass nut and pipe are frozen together. Hold large brass nuts while unscrewing small brass nut.

Squirt some Kroil or PB Blaster on it and let it sit for a day or two. then unscrew using two wrenches, keeping one side still while you turn the other. Tou'll want a pipe wrench for the galvanized pipe and a crescent or open end wrench for the nuts. Probably relatively big ones-- like 12" to get enough leverage. Be prepared to replace one or both sides of the joint if they are too corroded to come apart without damage. You could also drain the water from the line and then heat the joint with a torch, that's often very effective at loosening corroded threds.

That really doesn't look too bad, so it should come apart with reasonable persuasion.

The galvanized joint is the one you have to worry about, re-threading an installed pipe can be awkward and may require working backwards to a sound joint. Since you already have that kink in the copper I'd just cut it off and only worry about the joint to the galvanized pipe.

Thanks for the advice. Should’ve included the bigger picture. I’s easier to install this filter than it looks with those pics. Will put union on left, should I union right side or let copper fitting do that? Also the Copper pipe can swing left or right several inches.

Make sure that filter you are planning to use is capable (durable enough material) of being installed exterior.