Initially, I'm using the workouts that the trainer created. I have 12 workouts altogether, and I'm cycling through them with a goal of doing one per week, replacing one day of cardio. If I forget how to do something, or need to check on form, a quick search turns up plenty of examples.
This is not helpful for readers who may want to start strength training at home -- except to say that I'm very glad I finally worked with a trainer (covid silver lining: virtual options). I highly recommend doing this for a little while if you can afford it. I thought of it as an investment in my health, as I did when I bought my treadmill, similar to joining a gym.
The winning app: Nike Training Club
When I want more or a different challenge, I'm going to use Nike Training Club. I chose it because:
** It's simple and direct, not larded up with unnecessary features.
** It's focused on exercise only. That's all I wanted, and that's what NTC is.
** Workouts are clearly divided into beginner, intermediate, and advanced.
** There are programs grouped according to goals -- many of them, for a wide variety of goals -- or you can find a bunch of exercises that work for you and save them to create your own programs.
** You can also choose "whiteboard workouts" that combine exercises for a full workout on the level you choose.
** If you choose a whiteboard workout, each exercise includes a short optional video that demonstrates proper form.
** Most workouts require no or minimal equipment.
** Nike Training Club also happens to be free. I was willing to pay a reasonable amount for a workout subscription if necessary, but my first choice is free -- a nice bonus.
Other fitness apps
There are zillions of fitness apps. I used these articles to narrow them down: Forbes Health's Best Fitness Apps (recently updated) and Healthline's A Trainer's Picks of the 12 Best Fitness and Exercise Apps. Most of the other lists I saw are copies of these.
For me, most of these were easily eliminated, as they focus on needs that aren't relevant to me.
Many of the apps aim to be all-in-one health hubs -- diet tracking, exercise, lifestyle changes, coaching. I can see the appeal, but I can also see that easily overwhelming a beginner. In any case, I have those pieces under control, and I don't want to subscribe to something knowing that I'll ignore three-quarters of what it offers.
There are also many fitness apps for body builders, and for specific needs such as pregnancy. Many are designed for use with a wearable device (Fitbit, Apple, etc.) which I don't want and will never do.
All in all, Nike Training Club was an easy choice.
How often is enough
Many people believe that strength training must be done a minimum of three times per week in order to see results, but that's either a myth, or at best, not relevant to my goals.
My goals are the typical ones for older people -- google "why strength training is important for older adults" -- and are all about health and well-being. My long-term motivation is improving my chances of a healthy, independent old age. My shorter-term motivation is improving the ease of everyday movements and tasks. Strength training also feels good and, unlike cardio fitness, you can feel the benefits almost immediately.
Everything I've read about this kind of exercise says once or twice weekly is a solid goal. I found this article very helpful: A Low-Pressure Guide to Make Strength Training a Habit.