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Nothing should happen to your fields during migration as CB has little dependency on Joomla. If you uninstall plugins that previously had fields that could explain a few going away, but from a 100 to 34 shouldn't happen. How are you migrating info over as needed?

If this is a fresh install instead of an actual migration then you'll need to be sure to properly migrate all of your data manually, which can be complicated. Below are the database tables for all our products that you'd need to be sure are migrated to ensure your CB data remains.

_comprofiler
_cbsubs
_groupjive

This however is just CB data. Joomla user data is across multiple tables, but unless you migrated the install you probably won't be able to directly import data from these tables as they'd be incompatible so you'll need to modify the import or query to import or migrate the old install first. Below are the Joomla user tables.

_users
_user_keys
_user_mfa
_user_notes
_user_profiles

The users ACL assignments would also be related to the below tables.

_user_usergroup_map
_usergroups
_viewlevels

I did a completely new install of cb and cbsubs on j5.  I then just exported/imported the users.  That's all.
Wouldn't the fresh install include all those fields like address, city, phone etc? And, if they are, why don't they show up in Field Management? (complete list attached)

Wouldn't the fresh install include all those fields like address, city, phone etc?

No, a clean CB install does not include any fields beyond the minimum needed for CB to function. CB can generate sample data using the CB > Tools > Load Sample Data that does include some of those fields and CB Quickstart also includes those fields. Both are basically just creating those fields for you, but they are not core CB fields.

OK, my apologies for the goose chase. I have created my own fields I need and all is good!

I suspect some of the extra fields were from Virtuemart.