Don't just download random videos, focus on a specific niche like a dogs video or cat. They always go viral all the time because people love little pet.. Then after accumulating a lot of engagement and followers you can add Amazon affiliate links to ur bio or Shopify store that has pet-related stuffs like leaches, toys etc
It takes little to no effort to blow up on tiktok especially if your video are high quality.
Ps. I won't recommend to download video on tiktok and still post it on same tiktok, it won't go viral Imo, it better to download from Instagram or YouTube short and reload(look for a free tool online that you can use to reframe the size of the video to upload on tiktok)
Ps2. You can hire someone to create a review content for you if you want to promote a product and pin it on your tiktok with the link in ur bio, then focus on posting more content why it organic drive traffic to your pin video which will take them to bio.( don't promote too many product 3-5 is ok and pin the review video generated by someone you hire or yourself Aka UCG)
Ps3. Try and post particular/specific pet like dog or cat Breed, in order to look more like a friendly and natural account rather of stolen videos account.
Hope this helps.
Cheers