The excellent Stevie Grieve explains the difference between tactics (a more overarching philosophy that dictates how sides approach football regardless of opponent) and strategy (how within your tactical set up you plan on exploiting the weaknesses of a given opponent) on a recent World Football Index Tactics podcast. Most interestingly he discusses the tactics we've seen employed by each of the top 5 sides this season and how their strategies have changed based on game-specific circumstances.
He talks in depth in the first 12 or so minutes about how Ozil's role has changed with Alexis playing as our number 9 as opposed to Giroud. As we've discussed here at Soccermetrica, Grives points out that Sanchez drops into deeper positions and floats into the channels which encourages Ozil to make those runs in behind the defense in the space vacated by Alexis. Alternatively, when Giroud plays up front he stays central and higher up the pitch which encourages Ozil to move into deeper positions to get on the ball and create. It's no surprise then that we see Ozil with more goals and fewer assists than he had at the same point last season. Importantly, both strategies can be effective and allow Arsenal to be flexible in how they approach individual games based on the opposition and changing scenarios over the course of a match.
Around 18 minutes Grieve puts forth his thoughts on why Spurs went with a 3-5-2 against us (it allowed Son and Kane to press our two center backs, the wing backs to press our full backs and still left them with 3 v. 3 in midfield). It's a fascinating podcast and I highly recommend all of the WFI Tactics pods. Grieve delves deeper into game analysis than you'll get from just about any other source and does so in a way that's easy to follow and understand.