Park Tools are the best goto for instructional videos.
View: https://m.youtube.com/watch?v=eqR6nlZNeU8&vl=en
If you look at a rim it has a lower bit or well around the middle and the clincher bits that grip the tyre bead running around both edges.
https://en.m.wikipedia.org/wiki/Rim_(wheel)
The trick when fitting the tyre is to stuff and hold the wire bead of the tyre into the well ( starting near the valve. The smaller diameter of the well gives you more slack to pop the bead over the rim. Pinching the tyre all around with your hand can persuade the bead to drop into the well for more slack.
Also push the valve into its hole in the rim and seat the bead then pull the valve back out. Valves can stop the beads from seating properly.
Also check that the inner tube is not twisted and wont get pinched by the tyre. Putting a little air in makes this easier.
View: https://m.youtube.com/watch?v=eqR6nlZNeU8&vl=en
If you look at a rim it has a lower bit or well around the middle and the clincher bits that grip the tyre bead running around both edges.
https://en.m.wikipedia.org/wiki/Rim_(wheel)
The trick when fitting the tyre is to stuff and hold the wire bead of the tyre into the well ( starting near the valve. The smaller diameter of the well gives you more slack to pop the bead over the rim. Pinching the tyre all around with your hand can persuade the bead to drop into the well for more slack.
Also push the valve into its hole in the rim and seat the bead then pull the valve back out. Valves can stop the beads from seating properly.
Also check that the inner tube is not twisted and wont get pinched by the tyre. Putting a little air in makes this easier.