To expand on the above answer specifically to Susan's questions...
When you have Alaska miles and you want to use them on a Condor
flight, do you book them through Alaska or Condor? I have the same
question using Delta miles on KLM. Does help to call the miles issuing
airline directly? Thanks in advance for the clarification.
First, I don't think there's any reason to call any airline involved. Nobody is going to provide any useful help.
If you have Alaska miles, you can only use them to book on Alaska's website. When you book award flights there, you should find most options to book flights on Alaska fights, but also (some) flights on Alaska's partners. Whether or not those are good options (that is, a good value) is a separate question (with no simple answers) but the mechanics of using the Alaska miles are relatively straightforward. Use Alaska Miles on Alaska's website.
Same for Delta. Use your Delta miles on Delta's website. There you will find mostly Delta flight availability, but some (usually much more limited) options to book partner flights, too.
As a general rule (there are exceptions), miles/points from any airline's loyalty program can only be used when booking flights through that airline's system. They can't be transferred to other airline programs and used elsewhere. That said, most airlines do have partners (some have many) and (sometimes) their partner flights can also be booked, but you do that through the program where your miles reside (so in the examples above, use your Alaska miles at the Alaska website, your Delta miles at the Delta website, etc.).
Some cautions and caveats: If you're not well-versed at playing this game of using miles/points, it can be a complex mess, full of frustrating gotchas, terrible values, limitations and tricks, but also with plenty of opportunities and occasional pleasant surprises (those positive experiences are getting more challenging to find). There are also a lot of outright liars and absolute charlatans online (including one website shared right here every day) seeking to "monetize" you, under the guise of "helping you" through sorting it all out for you. Caveat emptor when following bloggers. If you want to play the game, the best places for true, accurate, and detailed info is FlyerTalk, but that's a jargon-filled swamp and it requires some dedication, effort and patience (more than most people care to invest) to be successful.
Re acraven's experience with United points and Turkish awards: availability and price of award flights on Turkish Airlines using United miles fluctuates wildly - occasionally they are cheap and plentiful, often they just disappear for months at a time, for no apparent reason. But IME it's worth watching them, when the fish are running, it's worth going fishing. Redeeming Turkish miles via the Turkish system is more complicated (and expensive) and fraught with bad IT frustrations, and not for the faint of heart.
Personally, I've never flown Condor (have flown on Alaska and their partners more than a few times using Alaska miles). I often book flights on United (and many of their partners) using United points. IME, United is probably the easiest system to use, with the most useful partners – though United devalued their awards tremendously last April, really gutting the value of their miles, so you better bring a wheelbarrow full of them for a good award redemption...they're still useful (I just finished booking flights with United miles to/from Europe for a trip next September, with the return flights on Turkish Airlines). IME United miles are generally the best option for flights to/from Europe. But I also find American miles to be often useful and occasionally a good bargain (though almost never on flights to Europe, still often useful for Latin America or Asia). YMMV.