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Can anybody explain how to work with thread priorities in OpenBSD?

Information about this question in MAN pages is poor, it does not give clear understanding. In particular:

sched_get_priority_min/max gives range. I have OpenBSD 6.3 amd64, and range is 0-31. 0 is lowest?

If i see default priority policy/level of first created thread of process, i see: (SCHED_OTHER, 0). If 0 is lowest, how can i specify idle thread? Or priority below normal? Can i only increase priority? Which priority have system threads? By idea, default priority level has to be normal.

MAN page pthread_setschedparam() says: "The scheduling policy for a thread can either be SCHED_FIFO (first in, first out) or SCHED_RR (round-robin)". Why not SCHED_OTHER? First default thread has this policy.

Can anybody make it clear?

I am newbie in OpenBSD. In Windows i can set idle priority, and i see that this thread is scheduled only when there is no other work in a system. I can set time_critical, and this thread has almost all processor time.

How to resolve these tasks in OpenBSD? What are analogues of Windows priorities: idle, lowest, below_normal, normal, above_normal, highest, time_critical?

How to make choice between policies? FIFO, OTHER or RR? Are these policies equal in granting processing time when priority level is the same?

A lot of questions without answers in MAN pages.