How can I add a 50AMP breaker for a NEMA 14-50 outlet to my panel?
The good news: you only have a few things to clean up in your panels
The good news in your situation is that your &quot;alien breaker&quot; problem is less severe than it appears at first glance; while your panels are Arrow-Hart (Murray), they are cross-labeled to accept Bryant (now Eaton) BR, Westinghouse (now Eaton) (H)QP, and ITE EQ-P (apparently now Siemens QP), in addition to being able to use UL classified Eaton CL breakers and their native Murray (now Siemens, but recently discontinued) MP breakers.  This means that we don't have to go on a wholesale breaker-swap rampage, but only have to fix a few troublespots.  In particular, since you have 4AWG copper going to the barn, we can change the barn subpanel breaker out for up to a 70A breaker, conservatively assuming that the wire is 60°C rated.  In other words, you can use a BR270, Eaton HQP2070, or possibly a Siemens Q270 here (the labeling issue is a bit weirder in this case since there appears to have been a designation change as the ITE breaker product line changed hands) if you can't find or don't want to search for a NOS Murray MP270 to go there, as Siemens discontinued the Murray product line over the course of the last couple of years.
Moving on to the micro-panel in the barn, we can start by removing the THQL1130 as it's no longer needed.  From here, your options depend on if the barn has a shutoff nor not; if there's a shutoff switch at the barn that cuts off all power to the barn, you can simply whack in a THQL2150 for the car charger and a TFH filler plate in the top right to plug the open hole.
If the barn does not have its own shutoff, though, then things get more complicated as you'll need to add such.  In that case, the filler plate in the top left comes out and both of the outer top slots are filled with THQP150 breakers, connected by a THT2 handle tie, to provide power to the car charger as it's a 240V only load and does not need common trip as a result, just common disconnecting.  From there, you'll want to add a THQL21100 (the precise rating isn't critical as long as it's greater than or equal to 70A as it's merely being used as a shutoff switch here) in the bottom spaces with a THQLRK retainer kit holding it down, then move the feeder wires from the main lugs to the THQL21100 to allow it to serve as a main shutoff for the barn power.  You should also make sure the bonding and grounding is correct here in this case; neither the welder nor the car charger need a neutral, so a 3-wire feeder with a single bonded bar in the barn subpanel can work, but you'll want to make sure you have a solid grounding electrode system at the barn all the same to keep stray natural electricity from making a mess there, at least.
Once that's done, we can move onto the rest of the situation.  The possibly-THQL? breaker feeding AC#1 should go and be replaced with a proper breaker for the panel, and the double-stuff breaker labeled &quot;Garage Plugs&quot;, while likely of a suitable type (Siemens QP/QT), looks to be overbreakered for the circuit unless it's feeding big, chunky 30A receptacles (NEMA 5-30 exists, and is dryer-receptacle sized) on 10AWG wire, of course.  Finally, we move onto the pool panel, where that Homeline HOM230 sticks out like a sore thumb in a BR panel, and needs to be replaced with a BR230.
Of course, if you want to upgrade either or both of the micro-panel subpanels to larger units, that is a fair prerogative; unless there's a safety-switch somewhere for the main cutoff for the barn, you'll want to use a 100A or 125A, main breaker, 24-space or 30-space panel in an outdoor-rated/weatherproof (NEMA 3R) enclosure there.  The pool panel looks to be at the same structure as the main panels, so the need for a main shutoff isn't an issue, and it can be upgraded to a 125A, 24- or 30-space, main lug panel in an indoor (NEMA 1) cabinet as a result.