I've been out of using 3d printers for about 2 years due to building a house, so I've forgotten all the terminologies used. Anyways when I had this issue on my ender 3 it was because I was using too much filament and the edges of each line was merging in to the next then pushing upwards slightly.

I had to really tinker with my first layer. What I had to do was make the initial layer a bit taller but also use slightly less filament. Sorry I can't go more in-depth with the terminologies.It worked though, I even got great first layers when using petg and wood based filament.