OS ads, website ads or app ads?

• For OS ads, it depends on the manufacturer. Some don’t have ads. For Xiaomi devices, ads can be completely removed by disabling a single app. Avoid anything by Samsung.
• For ads on websites, use Firefox with uBlock.
• App ads are the hardest to remove. You can try blocking ad domains at the OS level, or disable ads in each app’s settings (if they give you that option).