Most nuts and bolt heads have a washer under them. If you imprint the washer diameter on the surfaces of the mating parts, that is a good place to start.

Some models have a large number of bolted connections. Ansys has a tool to rapidly create a large number of connections by using a combination of Named Selections, using the Size checkmark and the Object Generator to automatically make copies of a Fixed Joint over all the items in a named selection. Here is an old video that demonstrates this feature:

If the parts being joined are thick solids, when you create a Fixed Joint and select the Reference side of the joint, the centroid of that area is where the joint coordinate system is placed. This means that the spider from the area on the Mobile side of the joint will reach up through the two thick solids to get to the coordinate system. This is not ideal. The ideal is to place the joint coordinate system on the joint axis at the mating plane. You can edit the origin of the joint coordinate system after the joint is created. I have a trick when making a joint to land the coordinate system in the ideal location. Set the object filter to Edge instead of Face and pick an edge of a hole in the mating plane as the Reference side. This will locate the joint coordinate system in the mating plane. Then change the object filter to Face and pick the imprinted washer area for the Mobile side and then click the Reference side to replace the edge with the imprinted washer area for the Reference side. Now you have a Fixed Joint where each spider reaches down to the mating plane.

After analyzing the stress in this model full of Fixed Joints, you might find that one or more of the joints is in a highly stressed location. That is when you can consider building a detailed model of that bolted joint. Delete the Fixed Joint. Add a solid model of the nut and bolt (without threads) at those holes. The nut and bolt head can be modeled as cylinders at the washer diameter. Add Frictional Contact between the mating surfaces. Split the shank of the bolt cylindrical face at the plane of the nut. Add a Bolt Pretension load to the bolt shank face. Add Bonded Contact between the nut and bolt shank that is inside the nut. Add Bonded (easier) or Frictional Contact (more accurate) between the imprinted washer faces and the nut and bolt head. Now you have a very detailed model that will be far more accurate for stresses around the bolted joint than the Fixed Joint.

Frictional contact is nonlinear while the Fixed Joint is a linear model. This means you may have to do extra work to get the solution to converge such turning on Auto Time Stepping and setting the Initital Substeps to 10 or 100 to get the convergence started.