Can someone else access my furnaces, personal quarry, or refinery?
Yes, unless you make sure they have TC coverage or place locks on them.
If you are only going to rely on TC coverage, you need to hold a building planner in your hand and walk around them. If you see "Building priv" then nobody can access something you placed right there.
It is smart to always put a lock on them which we do allow via a plugin. But you should always check all the way around an object. Sometimes a quarry hopper can be covered by build priv but not the fuel and someone can steal the fuel out.
Ultimately, never rely on a plugin to protect your belongings.