A lot of primary containers have been used in the past, ,, example clay amphora in Greek/ Egyptian times (pottery crocks), ,, wood vats in the Renaissance, ,, concrete tanks 200 years ago. Iron (galvanized), aluminum and copper tanks/ kettles are to be avoided. Iron with glaze is still done but check for cracks in the finish. Brass contains copper therefore brass fittings shouldn’t be used. Pottery currently is out of favor since some glazes contain lead, if 100 year old crocks are used they should be tested for lead.
Polyethylene/ HDPE seems to be used a lot now, as pails or brute trash cans or even 500 gallon plastic totes. I like PET big mouth bubblers since I can easily clean out pulp and put 120mm silicone covers on them (fruit wines). I also have some stainless stock pots and my dad’s 3 SS milking buckets.
The organisms used in fermenting pickles or kraut or kombucha will be killed with 150F water so I could see heat treating with boiling water if other fermentations were done. Any deeply scratched plastics present a risk of infection with past fermentations.
If you are fermenting fresh fruit, ALL spoilage organisms that ate typical will be in the fruit! Infections are controlled by creating barriers that select for yeast, ,, example pH and metabisulphite.