There are a number of popular methods for joining pipes, and the best method to use depends on the precise situation and environment.
Welding
Welding involves fusing together two or more parts using heat, with the joins forming as the parts cool down. There are a number of welding techniques, including arc welding, MIG welding, and TIG welding. The two main types of welds that are used to connect pipes are the butt weld joint and the socket weld joint. The butt weld joins two pipes of the same diameter, whilst the socket weld can also join pipes of different diameters.
Welding has some advantages over other methods, notably the strength of the join itself.
Soldering
Soldering involves using a special metal alloy (solder) to join the pipes together. The soldering iron is heated to a high temperature which allows it to melt and manipulate the solder, which is used to create the join as it cools down. Soldering creates strong permanent bonds and is a popular choice for joining copper pipes in particular.
Brazing
Brazing uses considerably higher temperatures than soldering to melt the filler material that is being used. As with soldering, the filler metal is used as an adhesive to essentially ‘stick’ the pipes together. This method is popular for joining pipes that transport material of medium temperatures. Brazed joints tend to be stronger than soldered ones but not as strong as welded ones.
Copper pipe connectors
One increasingly popular alternative way of joining pipes is to use specially designed connectors. These include press-fit options and are a great choice where heat-fusion methods are not practical. There are a number of specialists offering these connectors, and there are some detailed online resources available such as those seen here https://watkinspowis.co.uk/products/copper-pipe-fittings-and-press-systems.
Threading
Normally used in low-pressure systems, threading involves one component with internal threads and a connecting piece with external threads. This can be used on PVC, copper, iron, or galvanised iron pipes. High temperatures may cause these joints to expand/leak, so they should only be used for pipes with low-temperature flows.