Hello,
In Mechanical, if automatic contact detection is on (by default it is on), bonded contacts will be generated between parts that are touching. You can change this default setting if you don't want automatic contact detection from Tools > Options as shown below.
If you need parts to be connected rigidly, bonded contact is correct. For other parts where relative movement exist, you can delete the bonded contact and define the correct one, eg. frictional, frictionless, joints etc..
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