A current situation can have a past starting point
German present tense describes the situation as current. A phrase such as seit einer Woche tells you how long it has held. This can describe a habit or ongoing job, not necessarily an action continuing without a pause.
Nora arbeitet seit einer Woche im Museum.
Nora has been working at the museum for a week.
Seitdem has two sentence patterns
As a conjunction, seitdem begins a verb-final clause: seitdem Nora hier arbeitet. As an adverb, it occupies one position in a main clause and the finite verb comes next: Seitdem arbeitet Nora hier. The spelling is the same, but its job in the sentence is different. Seit can also introduce a subordinate clause.
Nora zog im Mai um. Seitdem fährt sie mit dem Rad zur Arbeit.
Nora moved in May. Since then she has cycled to work.
An endpoint is different from an earlier action
Waiting bis an event happens continues up to that event. Doing something bevor an event happens places the action earlier. Both connectors introduce verb-final clauses. In ordinary main clauses, keep familiar verb-second order.
Bevor das Museum öffnet, trinkt Nora einen Kaffee.
Before the museum opens, Nora has a coffee.
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References
Original explanations and examples informed by grammar references. Level placement is an editorial teaching choice.