The worst damage may be out of sight
A joist may feel firm in the middle of the room but be badly rotten where it rests inside the wall. If the floor has started dipping near an outside wall, that hidden end needs checking.
Not every joist needs total replacement
If decay is genuinely local and the remaining timber is sound, repairs may involve cutting back, strengthening or replacing only the damaged section. The correct method depends on loading, access and the condition of the surrounding structure.
Neighbouring joists should be checked
A shared moisture source can affect several bearings along the same wall. Finding one rotten joist end is therefore a reason to inspect the next joists rather than assuming the problem is isolated.
Drying and ventilation are part of structural repair
The timber repair will not last if the wall stays wet or the underfloor void cannot dry. Restoring airflow and stopping water getting into the building can be as important as the carpentry itself.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.