is it posible to buld a boiler for hot water use?

i want to learn so many things but i need s start point, please can someone help.

If the system is pressurized, then you will need to construct a tank which can tolerate the pressure without leaking. It should have a safety relief valve (according to standard). You need to install fittings which will accept an incoming and outgoing pipe.

It will be difficult to construct a tank that satisfies plumbing codes and regulations.

Or your system might be un-pressurized, which implies you have tanks delivering water from overhead by gravity feed.

You can heat the water by electricity, or by fire underneath the tank. Heating water by electricity takes a lot of power.

You should surround the tank with insulation to confine the heat.

If you get bright sun, it can assist in heating the tank.

There may be "hobby" exemptions for a boiler below certain
minimum size and/or pressure; this is very locale-dependent.

Hot water for residential uses does not need to be boiling,
so you might escape the regulations altogether if you are
able to show that temperatures or pressures are below
this point.

A neighbor of mine used to heat his above-ground pool
using a copper coil in an oil drum and burning scrap wood.
Garden hose for the rest of the run. Not very high temp
at all, but adequate to the task. Consider the tradeoff
between heat exchanger area, heated water flow demand
and working temperatures.

Water-tube boilers and fire-tube boilers I believe may
have different safety regimes / regulations / inspection
requirements due to their differing failure modes. But
the best key to regulatory escape, I think, is to avoid
boiling altogether.

The most complex part is water tank, so suitable to take it from anywhere rather then make it personally.
For instance recently i have done solar water heater with tank from old electric water heater.