To answer your question, look into how each platform calculates analytics. Different platforms have different methods for tracking analytics. Each platform has different criteria for tracking visitors and sessions, such as how long a returning visitor stays before it is counted as a new session. Attribution can also make a difference. For example, if someone visits your store from TikTok, leaves, and then returns via another link, you might attribute the conversion to TikTok (the initial source) or the second link (the actual converter). Opinions vary, and advertising platforms often have an incentive to credit themselves for conversions.
Shopify has documentation explaining why its analytics might differ from Google Analytics, which could be a good starting point. Additionally, check TikTok's attribution methods. This will help you understand the differences and choose the analytics approach that best suits your model for campaign decisions.