How do devices work together?

There are lots of posts and discussion about how the fitbit calculates a net amount with MFP. I'm no tech guru, but just comparing the numbers of on vs off, I can tell you this... if your activity is step-based (walking, running, stair climbing, even bike riding b/c of the stepping motion of pedaling) the fitbit does seem to track the activity accurately. I have done all these activities w/out my FB and with to see how the numbers compare, and they come out the same for the most part. One thing to note, the fitbit will sync up to your MFP account as much as you tell it to, and your MFP will adjust immediately to that activity - but when you close out your day it will say one amount remaining, and the next mroning it will be slightly less. This is because it takes a while for your MFP to catch up to the synced amounts, netted with the estimation that MFP makes about your other activity. I'm probably doing a terrible job at explaining or even understanding that,, but just wanted to give the heads up that if you're going totally FB, then leave yourself some wiggle room on the calories. I prefer this small amount of ambiguity because with an absolute 100% exact count/amount/measure I think it gives a false sense of security and too many reasons to slip. As with anything it's an estimation and weight loss is about moderation, and being active - it's not just a numbers game. So I like just letting FB do it's thing, and I do mine, w/out completely depending on FB to tell me how much I can have. Now, here's the kicker, I think if you do aerobic beyond walking, something that is more intense you are supposed to wear your FB +and+ put the activity in, and let MFP return with a net amount. I have done this some, but I mostly only do walking, so not much. Some things I just prefer to go sans FB b/c I like seeing that actual activity in my log. Like eliptical, dancing, swimming. And of course daily activities I always put in - gardening, cleaning, cooking, pushing a stroller. I mostly wear my FB, let MFP do the net # thing, and add any activities that are more than a basic level of exersion. I know that my walking is always at a dog walking pace, and that a 4.0 mph walk is actually a slow jog. I base it off that spectrum.

I use a Polar Loop paired with a H7 Bluethooth HRM and it solves what I need. Was the only fitness tracker I could find at the time that had a HRM built in and could handle HIIT Cardio and running.

exactly, whatever you choose, as long as you know it is to augment your own management of your progress, then you can't go wrong. It kind of gives me a guide of wheter I've been active or not in my day. Some days that active'ness is more casual movement (grocery shopping, socializing, housework) and sometimes it's more purposeful (actual cardio sessions). So I let it track what it will and anything extra is just icing on the weight-loss cake. Best of luck my friend. You're doing great.