Let me put this bluntly: a pro who wants to steal your bike can do so, and in a couple of minutes. That isn't to say you should ignore all precaurtions, not at all, as many bike thefts may be by kids, people who need the money, i.e. anybody but the pros. This was brought home forcibly when the unthinkable happened, and my bike was securely locked to a lamp post when the key snapped in half. By lucky chance I bumped into one of the council workmen, and, three minutes later, I was cycling the bike again. That three minutes included time to get the angle grinder, and drive to where the bike was parked. The lock was an Abu, not the top of the line; but you see the point. A pro has the right tools, and there's not a lot you can do.

Well there is. People like new bikes, so insurance is a good idea. However, as the bike ages, the worth decreases, to the point where insurance isn't worth continuing. That may take several years, especially if you buy a brand wth a good reputation. You should use the best lock you can anyway, but especially as your bike ages and the likely thieves descend the professional ladder and the insurance payout may not get you something of equal worth. The police may have schemes to register your bike, so go for them. Finally, look where you are leaving your bike, and always lock securely.

Once I had an expensive Trek MTB, my pride and joy. Took it to an unnamed, if notorious city for a tour, and, on the first morning, decided to drop into a café to buy some more mineral water. Bike was locked to a lamp post outside with two chains and a U lock. Went inside for a couple of minutes, but all that was left of my pride and joy when I departed were the locks. The next bike, bought a couple of days after, was a bright purple hybrid. Registered it with the police, who seemed rather amused by the colour and the thought one of their thieves might have the bad taste to steal it. I still have that bike, somewhat weathered now, never insured. It raises eyebrows when I take it for a spin.
In the end, as mentioned, a professional who wants your bike is probably going to get it. However, many are not, especially now the scooter/electric bike market is taking off.