How to do, in clear steps tutorial, all the works of firmware updating / refreshing so fixing it while get better newer version at once
Every phone is different, with different flashing steps, which is why you were asked which phone you have. Though, I personally think your best option will be to replace it with something newer.
For one, it sounds like you are new to performing your own diagnostics and repairs, and something like re-flashing the stock firmware carries a risk of completely bricking your phone. Second, Android 6 is severely obsolete. Re-flashing the existing firmware would still have many issues even if it is firmware corruption causing the bootloop (and no guarantee it is). Any phone running that would also be long past its update cycle and not likely to have a new enough version to be worth the hassle.
If you are just absolutely tied to this specific phone, I would take it to a repair shop. The downside to this is it'll likely cost more than it's worth to repair it. Parts may also be hard to come by if it is hardware related. Regardless if it's firmware or hardware, you are almost certain to lose any personal data in the process of repairing it.
For the cost of repair, you'll likely find a newer and better phone for a similar price. Even if it's used, it shouldn't be hard to find one that's at least currently supported with security updates without spending a lot of money.