Two tools in the field editor control when each form field appears.
Conditions between fields
A field can show only if a condition on another field is met: you pick the source field and the value it must have.
Example: show "T-shirt size" only if the participant ticked "I want a t-shirt"
A field can have several conditions, defined in the field editor's Conditions tab
On saving, half-built conditions (with no source field) are discarded. If you change the source field, review the condition's value: the previous field's values no longer make sense.
Fields per fee
In the field editor's Fees tab you choose which fees (individual or team) the field appears in. With no selection, it shows in all.
Example: ask for "Federation license" only in the federated fee
Example: the "Bus" field with extra cost, only in the fees of venues with a bus
Tips
Combine both: a field can be limited to some fees AND depend on a condition
Less is more: every unnecessary field costs conversions
Test the result with a trial registration on each fee
The rest of the field editor (types, required flags, extra costs) is covered in Registration form.