Understanding bluetooth behavior

I am using Ubuntu 18.04 with Mate 1.20.1 on Lenovo Ideapad.

Some of my BT devices connect to the system automatically (if they are paired previously), but some don’t even if they are paired previously. Then I have to connect them manually by opening the blueman-applet. Example: BT speaker connects automatically, Smartphone doesn’t, Wireless headset sometimes doesn’t, Tablet doesn’t. In all these cases, BT is activated on both sides. Sometimes I can transfer a file to/from phone to laptop, sometime it is ‘unsuccessful’ without giving any reason…

Also, if BT is active on both the laptop and phone, then my handsfree connects to the phone correctly and not to the laptop. I wonder why it doesn’t choose the laptop.

I am trying to understand how BT is expected to behave with Linux and other devices and what diagnostic tools are available to check if it is configured correctly. I have limited technical knowledge, so many web articles apear too technical and cryptic. Also, there seems to be a great variety of issues/environments, so determining what to do with my own setup can often be tricky trial and error.

So looking for some tips on good resources to read for understanding how BT behaves (under Ubuntu/Linux) and how to check if it is set up properly and doing what it should on my setup.