I think the main downside people don't realize about apps on your phone is that by downloading them, you're giving a company an incredible amount of permissions over your phone and it's content. Read through say TikTok or Facebook's (for example) TOS for their app. When you download their app and sign in, they have the LEGAL RIGHT to go through your other unrelated photos on your phone, including nudes, documents, screenshots (think financial documents, passwords, etc) absolutely EVERYTHING. Not only to peek, but to SAVE.
And the app for Tik Tok specifically gives the Chinese company to not only go through your phones location, photos, and other info, but also ANYONE ELSE who is even on your same network (say they connect to your home wifi). So even if I'm super careful and don't even download anything other than Apple apps and maybe a bank app and Netflix or so, my daughter up in her room with the TikTok app is giving them permission to go through my phone anyway. It's absolutely ridiculous.
Besides permissions to data, they use your wifi and battery to send this info to their servers. On average, the 2 days I had it downloaded, the Facebook app took up 60% of my daily battery, and that's while the app wasn't even opened. What are they doing with my connection and all that power? Harvesting data and current info from my phone, at my expense.
To this day I don't know why there aren't people making a huge fuss over this, or that someone doesn't make it their mission, or job (or YouTube channel) to put this home invasion type behavior into plain words so the average person can understand how invasive and sneaky/dishonest it is.