what app do you use to protect your phone?

Guys, I really need your help!
I am using XIAOMI 4 and this phone does not have a built-in lock.
I need to lock my photos and videos because I sometimes need to show pictures to my colleague and they just had the chance to saw all of my photos.I don't have anything weird in my gallery but it's just felt strange that someone just go through all your photos.
I heard there are some good apps designed for lock apps, but I have never used any of them before. Especially some of them just lock all...

what app do you use to protect your phone?

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