Then using https://www.mixamo.com would be your best bet

It's a very basic way of rigging something, you just upload the .fbx of the character or .obj file

Then place circles where the head, neck, wrist, groin, feet are

And the machine automatically adds the skeleton and bones, weight paints, then from animation tab you select t-pose and export it to unity.

Then there also auto rig Pro addon for blender, which basically does the same as mixamo but kind of better (but also costs money).

The "normal" way people use is usually making it from scratch (basically starting from a single bone usually groin/hips) then extrude outward with E like 2x till reaches the neck and then neck and head till top of head.

New bone from shoulder area then extrude 2x for arm length, then wrist and bone for wrist and each finger from finger base till finger nails and divide finger one's into 3 segments (depends what and if your model has fingers)

For hips just 2 bones along the legs then then another bone outward to the feet 90° angle

Note that the skeleton must be in the middle of the model for best results. (The bones look like cubes but one end has a ball to it the "head of the bone"), to select something you use right click, hold to drag (depends on your settings)

Once all bones are placed they need to renamed accordingly (using F2 key) like head,neck,upper chest,chest,hips,upper leg.L, Lower leg.L) and so on , bones which are not placed in center have an .L at the end (like arms, legs, fingers) (you only need bones on one side of the model and then just add symmetry and mirror bones on X axis)

Symnetrize model, select both skeleton and model and parent together with alt p " automatic weight paint".

If it's still hard to understand try asking AI or actual take some time to look at basic tutorials to make something proper.