Yeah, there's always a learning curve getting used to something new, and don't forget smartphones are pretty complicated and even once you do get comfortable with it, some update/upgrade could change the user interface enough that you need to re-educate yourself all over again.
Also, when you install an app, or even with the ones that come pre-installed, be sure to dig into the Settings menu of each app. There's often a lot of options that might benefit you there too. Oh and one thing that's kind of bizarre is there's no default name nor location for app 'settings' menus. Might be named 'settings' but some developers prefer to use 'options' or 'preferences'; and to add additional vagueness, with some apps that settings menu might be designated by or just be a sub-menu in either a three horizontal dots icon, or a three vertical bars (a.k.a 'hamburger'), or a gear icon in either the top, bottom, left, or right corner. So just be prepared to do a little sleuthing looking for the menu, a good thing being you don't typically need to access the settings menu very often as it's more of an initial set up/configuration thing.
Your choice for a OnePlus 6T should work out well for you. Looking online it's getting some really good reviews.