There are several options, applied as they wish by the institutions.
These include (not an exhaustive list as based on experience from places I have been):
1) extra time ie 20% or more longer.
2) separate room - fewer people ie quieter.
3) a « coach » does not provide answers but helps student « think » and structure response.
4) a scribe, who writes the answer but does not provide the answer, just transcribes the words of the student.
5) providing "oral" based exams ie questions prepared to read to the student and responses written for the student.
The choice of strategy depends on an assessment of the student. This assessment is usually completed by a "Student Learning Officer" or equivalent, who can assess the student’s learning impairment & needs and decide accordingly.
Some of these require a huge effort for implementation ie option 5 means the professor gets to write a unique exam which takes a lot of time...