Well, there ARE standards. Just so many of them!!
"Pipe" is measured by the diameter of the hole, for capacity. But a stronger pipe is thicker pipe, needing bigger "standard" pipe fittings.
"Tube" is measured by the diameter of the media. The fittings may stay the same/similar size. but stronger sized tubing flows less.
Older houses are an expletive at times. Galvanized pipe coming in *needs* to be replaced every @ 15 years). Connecting to copper tubing to feed appliances (sinks, washers, toilets, etc) Cast iron drainage pipes (out to sewage, etc). Depending on component alloys and water quality, electrolysis can be a big concern. Most times at the connection, but sometimes also at a grounding point. Acidic water with high iron will eat copper- better get plastic. You might need a surge tank at the beginning, and/or air relief at the top.
I worked on my father-in-law's house. All the above! Including a newish sprinkler pipe bathroom in the basement. I spent a weekend just tracing everything instead of patching and repairing. Got him a "good deal" (I paid half up front!) on a full copper repipe. Magic- everything suddenly worked- full pressure- and they sold after ten years of no problems.
EVERYtning converted to shedule80 tubing.